Bump WordPress tested up to plugin's header tag to 6.4

[imath]

Just tested BuddyPress 11.4.0-beta1 with WordPress 6.4-RC2 and it seems to behaves nicely with it.

This is a note to self, so that I think about bumping this header tag before releasing 11.4.

[imath]

In 13622:

Bump "Tested up to:" & "Stable tag:" readme header tags to 6.4 & 11.4

This commit also adds changelog & upgrade notice information for BP 11.4.0.

See #9012 (Trunk)

[imath]

In 13623:

Add missing changelog & upgrade notice information for BP 11.3.2.

See #9012 (Trunk)

[imath]

In 13624:

Bump "Tested up to:" & "Stable tag:" readme header tags to 6.4 & 11.4

This commit also adds changelog & upgrade notice information for BP 11.4.0.

Fixes #9012 (Branch 11.0)