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

## Summary

This PR updates the documentation to reflect the new minimum requirements introduced in BuddyPress 15.0.0.

### Changes made

• Updates the minimum supported PHP version to 7.0.
• Updates the minimum required WordPress version to 6.4.
• Adds recommended versions (PHP 7.4+, MySQL 8.0+ or MariaDB 10.5+, HTTPS).
• Explains why the requirements were raised.

### References

• ​Trac ticket #9051
• ​BP Team Updates blog post

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Fixes: buddypress/bp-documentation#295