Raise WordPress required version to 6.1

[imath]

As explained ​here, I suggest to raise the WP required version to WordPress 6.1.
At first I thought of raising it to 5.9 (as some important block theme functions were introduced in it), but it's only ​2.1% of active installs.
Then I thought about raising it to 6.0 but choosing 6.1 would allow us to simplify GitHub actions and remove a dev dependency to wp-phpunit composer package. See #9053

[espellcaste]

Good addition!

• 6.1 required WP version bumps
• Remove checks for WP versions < 6.1
• deprecate 2 BP Block functions

Trac ticket: ​https://buddypress.trac.wordpress.org/ticket/9051

[imath]

In 13718:

Raise WordPress required version to 6.1

• Do required WP version bumps.
• Remove WP function/version checks when these were about WP version < 6.1.
• Init the 14.0.0 deprecated functions file.
• Update the GH PHP Unit Action and remove the 6.0 wp-phpunit/wp-phpunit composer package downgrade that was needed when testing WP version < 6.1.

Fixes #9051
Closes ​https://github.com/buddypress/buddypress/pull/226