Align Synapse version string to use SYNAPSE_VERSION

[MadLittleMods]

See matrix-org/synapse#12973 where we previously used version_string="Synapse/" + get_distribution_version_string("matrix-synapse") everywhere; and then updated to use version_string=f"Synapse/{SYNAPSE_VERSION}" for every other place except synapse/app/homeserver.py (why?!?!?!). This seems more like a typo than something on purpose especially without any context in the comments or PR. The whole point of that PR was trying to solve the missing git info in version strings.

For reference, here is what both variables look like for me locally on the latest develop:

• SYNAPSE_VERSION: 1.139.0 (b=develop,1d2ddbc76e,dirty)
• VERSION: 1.139.0

Only reason we may want to do this is to hide the branch name (some sensitive name that exposes a security fix, etc). But we don't hide anything:

https://matrix.org/_matrix/federation/v1/version

{
  "server": {
    "name": "Synapse",
    "version": "1.139.0rc3 (b=matrix-org-hotfixes-priv,f538ed5ac3)"
  }
}

On matrix.org, the Server response header is masked as cloudflare which would otherwise show 1.139.0rc3 for everything from the main process.

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This is spawning from looking at the way we setup and start Synapse for homeserver tenant provisioning in the Synapse Pro for Small Hosts project (https://github.com/element-hq/synapse-small-hosts/issues/221)

Pull Request Checklist

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[MadLittleMods]

Exactly the same as every other spot where we specify the version_string in the Synapse code base.

[anoadragon453]

Seems sensible to me, looks like it just got missed. Thanks for fixing!

[MadLittleMods]

Thanks for the review @anoadragon453 🐢