Update MSC4461 support to v3

[blueberrymuffin3]

Description

Update MSC4461 support from v2 to v3. Also adds support for the keep_trigger option when sending messages (not yet in settings)

See #1279

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

[blueberrymuffin3]

My one question is if we want to delete the v2 account data like we did for the sable -> msc4461 migration.

[blueberrymuffin3]

I pulled out the part of the migration that cleans up the v2 data into #1930 so it can be merged in the next stable release.