Release 1.6

[aignas]

Creating a blank issue to keep track of things.

Things to merge before first RC:

• feat: freethreaded support for the builder API #3063

Process checklist:

• docs: update for 1.6 release #3205 Docs update
• create a new release/1.6 branch
• rc0 release and communication
• 1 week testing
• Cherry-pick fix(gazelle): Do not build proto targets with default Gazelle #3216
• Cherry-pick fix(pypi): add __init__.py to site-packages directories #3220
• rc1 release and communication
• ...
• Final release

[rickeylev]

1.6.0rc0 has been released.