Switch to Identity Service API v2

This issue has been migrated from [#9677](https://github.com/matrix-org/synapse/issues/9677).

---

The v1 Identity Service API will be [dropped from the spec soon](https://github.com/matrix-org/matrix-doc/pull/2713). There are a number of places where we still use v1 APIs, where we should instead first try v2, then fall back to v1.

https://github.com/matrix-org/synapse/blob/e550ab17adc8dd3c48daf7fedcd09418a73f524b/synapse/handlers/identity.py#L124-L137

https://github.com/matrix-org/synapse/blob/0a00b7ff14890987f09112a2ae696c61001e6cf1/synapse/handlers/identity.py#L273-L279

https://github.com/matrix-org/synapse/blob/0a00b7ff14890987f09112a2ae696c61001e6cf1/synapse/handlers/identity.py#L446-L456

https://github.com/matrix-org/synapse/blob/0a00b7ff14890987f09112a2ae696c61001e6cf1/synapse/handlers/identity.py#L499-L508

https://github.com/matrix-org/synapse/blob/0a00b7ff14890987f09112a2ae696c61001e6cf1/synapse/handlers/identity.py#L586-L595

I believe we also store IS URLs (`/_matrix/identity/api/v1/pubkey/[ephemeral/]isvalid`) in third-party invite state events, which will eventually fail once IS's drop `v1` APIs. Edit: Yes, we do:

```json
{
"type": "m.room.third_party_invite",
"sender": "@andrewm:amorgan.xyz",
"content": {
"display_name": "h...@g...",
"public_keys": [
{
"key_validity_url": "https://vector.im/_matrix/identity/api/v1/pubkey/isvalid",
"public_key": "ta8IQ0u1sp44HVpxdFOdS/bfwDjcy4xLFFlfY5KOA"
},
{
"key_validity_url": "https://vector.im/_matrix/identity/api/v1/pubkey/ephemeral/isvalid",
"public_key": "unYLHHHx_-kYDh9RLh5RvfcTrDgWnNtLdgPC3yM"
}
],
"key_validity_url": "https://vector.im/_matrix/identity/v2/pubkey/isvalid",
"public_key": "ta8IQ0u1sp44HVpxYi7dFOdS/bfwDjcy4xLFlfY5KOA"
},
"state_key": "HgjJXrYyxxxOtdmyiJHyYRTjMMAiFdDrbCPqyCMdgCSyGMvxsyztxNQHBXMcUOnLjUOdhCMzSzwYEowxPqIYGYcOnXzNMvAlyOKOmnWrnfglNfBAeVfmZLBvQRqDGg",
"event_id": "$161658804355dhuce:amorgan.xyz",
"origin_server_ts": 1616588043476,
"unsigned": {
"age": 6048
},
"room_id": "!xxx:amorgan.xyz"
}
```

---

Update 2022/06/24: It's also worth noting that we use the v1 `/store-invite` and `pubkey/isvalid` APIs when clients send a 3pid invite with no `id_access_token`, which we need to disable:

https://github.com/matrix-org/synapse/blob/d54909956ef616d976b3d9969be994df5b65030a/synapse/handlers/identity.py#L893-L911