Legacy Widgets: next step before deprecation

[imath]

In 9.0.0 we've added 1 Block Widget for each Legacy Widget. Block Widgets are including a specific function to transform the corresponding Legacy Widget to the Block one.

The downside of this is shown into the above screenshot, when looking for "buddypress" Blocks into the widgets screen introduced in WordPress 5.8, only Legacy Widgets are listed.

Users are able to migrate Legacy Widgets to Block ones in BP 9.0.0. In 10.0.0, I suggest to take a new step before deprecating Legacy Widgets (when our required WP version will be >= 5.8). When the theme supports Block Widgets and WordPress is >= 5.8, Legacy Widgets will not be loaded unless a specific filter is used to retain them using this code:

add_filter( 'bp_core_retain_legacy_widgets', '__return_true' );

What's the opinion of the team about it?

[imath]

I've just tested with the Classic Widget Editor. I confirm Legacy Widgets will be available for people using this plugin.

[imath]

In 13153:

Only load BP Legacy Widgets when the Widgets Block editor is disabled

To improve the Widgets Block editor's users experience, we are making a new step towards deprecating BP Legacy Widgets by not loading them by default. As a result, into this editor, only BP Widget Blocks will be loaded and listed into the Block inserter of the Widgets screen.

If users want to keep the Legacy Widgets into the Widgets Block editor, they can do so returning true to this filter 'bp_core_retain_legacy_widgets'.

If users are using the Classic Widgets Editor plugin or if advanced users are directly filtering 'use_widgets_block_editor' to return false: Legacy Widgets will still be available into the Classic Widgets Editor.

Fixes #8594