Remove the experimental flag for the Block Directory

[tellyworth]

Is your feature request related to a problem? Please describe.
The Block Directory is currently only enabled via an experimental flag. That means it's seeing only minimal testing.

Describe the solution you'd like
The flag should be removed, enabling the Block Directory by default.

[ellatrix]

Cc @TimothyBJacobs I think here as well, we can remove the experimental namespace on the endpoints and stabilise next release? Not sure if this feature will need adjustments after launch.

[TimothyBJacobs]

We need to land #23219 first before changing the namespace. I'm also going to push a change to change the plugin field to not have .php at the end.

[TimothyBJacobs]

I've opened #23395 for the plugin field change.

[ellatrix]

Fixed by #23389.