What problem does this address?

As __experimentalDefaultBlock and __experimentalDirectInsert are in use across a few blocks now and have been in use for some time it's agreed that it's time to stabilize these APIs.

The big driver for this is to support the inclusion of defaultBlock as an attribute starting with the core/navigation in #50982.

What is your proposed solution?

Stabilize the __experimentalDefaultBlock and __experimentalDirectInsert by completing the following:

• Remove __experimental prefix in codebase and replace with defaultBlock and directInsert.
• Add deprecated calls where existing implementations are found.
• Write small doc explaining usage and usage of associated other api's/selectors like __experimentalGetDirectInsertBlock