Framework: Deprecate gutenberg_get_post_from_context

[aduth]

Previously: #21467 (#21467 (comment))

Context:

#21467 adds first-class support for block context, which by default includes support for providing the post ID to any block which defines an intent to consume this context value.

This had previously been achieved by the implementation of a gutenberg_get_post_from_context function.

Task:

1. Replace existing occurrences of gutenberg_get_post_from_context to use block context
   • See Post Title block implementation for prior art
2. Add _deprecated_function function call in gutenberg_get_post_from_context.
3. Add entry to deprecations.md for two versions in the future of when the deprecation is added.
4. (Optional) Move gutenberg_get_post_from_context to a more appropriate file. compat.php is intended to be reserved for functions with a corresponding WordPress proposal ticket proposing the addition of a function.
5. (Future) Remove the function after the deprecation period has elapsed