Removal: Remove support for experimental msc3886

[rahulporuri]

fixes #17619

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[anoadragon453]

This LGTM on the whole, thank you!

Lastly I didn't mention this in my original comment. But despite this being an experimental feature, I think we should put a paragraph in the upgrade notes letting sysadmins know that this option is going away. The upgrade notes are typically where we warn of breaking changes.

So something like:

# Upgrading to v1.115.0

## Removal of experimental MSC3886 feature

[MSC3886](https://github.com/matrix-org/matrix-spec-proposals/pull/3886) has been closed (and will not enter the Matrix spec). As such, we are removing the experimental support for it from Synapse in this release.

The `experimental_features.msc3886_endpoint` configuration option has been removed.

[anoadragon453]

Hi @rahulporuri, are you willing to finish this one off? It doesn't need much more!

[rahulporuri]

sorry for the radio silence @anoadragon453 , will wrap things up tomorrow/weekend. I hope that's okay. I'll update this and my other PR too.

[anoadragon453]

Certainly! Thank you :)

[anoadragon453]

Hi @rahulporuri, do you have time to finish this PR? Otherwise I'm happy to take it over to get it over the line. Thanks!

[rahulporuri]

please take over @anoadragon453 🙏 I haven't been able to make time and I doubt I will be able to discover any free time this weekend either :/ . I sincerely apologize for not being able to get this past the finish line

[anoadragon453]

That's alright @rahulporuri, life happens! The team is grateful to you for kicking off this effort in the first place 🙂

[CLAassistant]

All committers have signed the CLA.

[anoadragon453]

This is now ready to merge!