Dependency update breaks OIDC for Lemonldap

This issue has been migrated from [#16166](https://github.com/matrix-org/synapse/issues/16166).

---

### Description

### PROBLEM
Upon updating from synapse v1.89.0 to synapse 1.90.0, the home server stops at startup on 504 No response error during OIDC configuration setup.

### ANALYSIS
Well I checked the code that changed for OIDC metadata loading since 1.89.0 : nothing absurd.
Then I thought of verifying whether a certain library version update might have caused the issue, and indeed : only one single dependency change is responsible for the error, that of `service-identity` which changed from `version = "21.1.0"` to `version = "23.1.0"` in `poetry.lock`

### SOLUTION
Revert `service-identity` to `21.1.0`, and lock the dependency version to less than 23.x.x
And if you're motivated enough, try to figure out why the dependency version change caused this issue.

### Steps to reproduce

- have docker (24.0.50), npm (9.2.0), node (18.13.0), and docker-compose (2.18.2) installed.
- clone [this repository](https://github.com/jcabannes/synapse-with-oidc-crashing) (thanks @jcabannes !)
- npm install
- npm run start:debug
- Take note of synapse's docker service logs.

To see the difference, modify synapse's version from 1.90.0 to 1.89.0 in `__test-data__/docker-compose.yml`, the error should disappear.

### Homeserver

local test

### Synapse Version

1.90.0

### Installation Method

Docker (matrixdotorg/synapse)

### Database

SQLite

### Workers

Single process

### Platform

Debian 12 stable, amd64

### Configuration

in homeserver.yaml
```
server_name: "example.com"
public_baseurl: "https://matrix.example.com/"
pid_file: /data/homeserver.pid
listeners:
- port: 8008
tls: false
type: http
x_forwarded: true
resources:
- names: [client, federation]
compress: false
database:
name: sqlite3
args:
database: /data/homeserver.db
log_config: "/data/matrix.example.com.log.config"
media_store_path: /data/media_store
registration_shared_secret: "u+Q^i6&*Y9azZ*~pID^.a=qrvd+mUIBX9SAreEPGJ=xzP&c+Sk"
report_stats: false
macaroon_secret_key: "=0ws-1~ztzXm&xh+As;7YL5.-U~r-T,F4zR3mW#E;6Y::Rb7&G"
form_secret: "&YFO.XSc*2^2ZsW#hmoR+t:wf03~u#fin#O.R&erFcl9_mEayv"
signing_key_path: "/data/matrix.example.com.signing.key"
trusted_key_servers:
- server_name: "matrix.org"
accept_keys_insecurely: true
accept_keys_insecurely: true
app_service_config_files:
- /data/registration.yaml
oidc_config:
idp_id: lemonldap
idp_name: lemonldap
enabled: true
issuer: "https://auth.example.com/"
client_id: "matrix1"
client_secret: "matrix1*"
scopes: ["openid", "profile"]
discover: true
user_profile_method: "userinfo_endpoint"
user_mapping_provider:
config:
subject_claim: "sub"
localpart_template: "{{ user.preferred_username }}"
display_name_template: "{{ user.name }}"
```

### Relevant log output

```shell
Error during startup:
Traceback (most recent call last):
File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 140, in load_metadata
await p.load_metadata()
File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 588, in load_metadata
return await self._provider_metadata.get()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/synapse/util/caches/cached_call.py", line 136, in get
return await self._cachedcall.get()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/synapse/util/caches/cached_call.py", line 106, in get
self._result.raiseException()
File "/usr/local/lib/python3.11/site-packages/twisted/python/failure.py", line 504, in raiseException
raise self.value.with_traceback(self.tb)
File "/usr/local/lib/python3.11/site-packages/twisted/internet/defer.py", line 1693, in _inlineCallbacks
result = context.run(
^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/twisted/python/failure.py", line 518, in throwExceptionIntoGenerator
return g.throw(self.type, self.value, self.tb)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/synapse/util/caches/cached_call.py", line 126, in _wrapper
return await f()
^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 600, in _load_metadata
metadata_response = await self._http_client.get_json(url)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/synapse/http/client.py", line 559, in get_json
body = await self.get_raw(uri, args, headers=actual_headers)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/synapse/http/client.py", line 644, in get_raw
response = await self.request("GET", uri, headers=Headers(actual_headers))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/synapse/http/client.py", line 411, in request
response = await make_deferred_yieldable(request_deferred)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/twisted/internet/defer.py", line 892, in _runCallbacks
current.result = callback( # type: ignore[misc]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.11/site-packages/synapse/http/client.py", line 952, in _timeout_to_request_timed_out_error
raise RequestTimedOutError("Timeout waiting for response from remote server")
synapse.http.RequestTimedOutError: 504: Timeout waiting for response from remote server

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/usr/local/lib/python3.11/site-packages/synapse/app/_base.py", line 254, in wrapper
await cb(*args, **kwargs)
File "/usr/local/lib/python3.11/site-packages/synapse/app/homeserver.py", line 362, in start
await oidc.load_metadata()
File "/usr/local/lib/python3.11/site-packages/synapse/handlers/oidc.py", line 144, in load_metadata
raise Exception(
Exception: Error while initialising OIDC provider 'oidc-lemonldap'
```

### Anything else that would be useful to know?

_No response_