Tag messages don't include changelog headers

This issue has been migrated from [#16694](https://github.com/matrix-org/synapse/issues/16694).

---

Before:

```
$ git show v1.86.0 | head -n 20
tag v1.86.0
Tagger: Mathieu Velten
Date: Tue Jun 20 17:23:48 2023 +0200

Synapse 1.86.0 (2023-06-20)
===========================

No significant changes since 1.86.0rc2.

Synapse 1.86.0rc2 (2023-06-14)
==============================

Bugfixes
--------

- Fix an error when having workers of different versions running. ([\#15774](https://github.com/matrix-org/synapse/issues/15774))

Synapse 1.86.0rc1 (2023-06-13)
==============================

```

After:

```
$ git show v1.87.0 | head -n 20
tag v1.87.0
Tagger: Olivier Wilkinson (reivilibre)
Date: Tue Jul 4 16:35:04 2023 +0100

Please note that this will be the last release of Synapse that is compatible with
Python 3.7 and earlier.
This is due to Python 3.7 now having reached End of Life; see our [deprecation policy](https://matrix-org.github.io/synapse/v1.87/deprecation_policy.html)
for more details.

- Pin `pydantic` to `^1.7.4` to avoid backwards-incompatible API changes from the 2.0.0 release.
Resolves https://github.com/matrix-org/synapse/issues/15858.
Contributed by @PaarthShah. ([\#15862](https://github.com/matrix-org/synapse/issues/15862))

- Split out 2022 changes from the changelog so the rendered version in GitHub doesn't timeout as much. ([\#15846](https://github.com/matrix-org/synapse/issues/15846))

- Improve `/messages` response time by avoiding backfill when we already have messages to return. ([\#15737](https://github.com/matrix-org/synapse/issues/15737))
- Add spam checker module API for logins. ([\#15838](https://github.com/matrix-org/synapse/issues/15838))

- Fix a long-standing bug where media files were served in an unsafe manner. Contributed by @joshqou. ([\#15680](https://github.com/matrix-org/synapse/issues/15680))
- Avoid invalidating a cache that was just prefilled. ([\#15758](https://github.com/matrix-org/synapse/issues/15758))
```

I'm guessing there was a towncrier update between v1.86.0 and v1.87.0 which changed the headings style.