Swap to using github action to configure synapse server in CI builds

[michaelkaye]

Type of change

• Feature
• Bugfix
• Technical
• Other :

Content

Rather than use a shell script to start and stop synapse, use a github action instead

Motivation and context

The synapse demo script changed which caused CI to fail, so fixing it up so it's internal to GHA rather than re-using a script not quite designed for this purpose.

Additional benefits is that this should retain the configuration and logs of the synapse homeserver for analysis after the run.

Tests

Tested by manual runs

Checklist

• Changes has been tested on an Android device or Android emulator with API 21
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request includes a new file under ./changelog.d. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR
• If you have modified the screen flow, or added new screens to the application, you have updated the test UiAllScreensSanityTest.allScreensTest()

[github-actions]

Unit Test Results

  96 files  ±0    96 suites  ±0   1m 7s ⏱️ -3s
172 tests ±0  172 ✔️ ±0  0 💤 ±0  0 ❌ ±0 
564 runs  ±0  564 ✔️ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 70fbcec. ± Comparison against base commit fbc3b15.

♻️ This comment has been updated with latest results.

[bmarty]

LGTM, the CI seems happy too. Thanks!