Block Bindings: Use `field` instead of `key` for post-data and term-data sources.

[cbravobernal]

The first use case for block bindings was post-meta source. This one keeps a key - value relation, which makes totally sense. But all examples for Block Bindings have been done with this key argument, and the UI was pretty key dependant.

After the UI refactor, args can contain anything, and using the argument that fits better for each source is encouraged.

For post-meta and term-data, key may not be the best, as it will connect in a future to different fields.

The source has been introduced during 6.9 cycle, but lived inside Gutenberg for a couple of versions, so I added backward compatibility with key only for the Gutenberg plugin, that way we cover:

WP 6.8.X and 6.7.X versions that used the new post-date approach with Gutenberg enabled.
WP 6.9 without GB will use the migration to the new post-date with field.
WP 6.9 with GB enabled.

Trac ticket: https://core.trac.wordpress.org/ticket/64112

Backport for ​https://github.com/WordPress/gutenberg/pull/72382.

[cbravobernal]

In 60955:

Block Bindings: Use field instead of key in post-data source.

After the UI refactor, source args can contain anything, and using whatever fits better for each source is encouraged.
Instead of using a post-meta based key argument. post-data will use field as an argument, which fits better to post related use cases.

Props cbravobernal, bernhard-reiter.
Fixes #64112.

https://core.trac.wordpress.org/changeset/60955 moved post-data key args to a better fit field one. I forgot to include link, noted in ​https://github.com/WordPress/gutenberg/pull/72382#discussion_r2440578117

Trac ticket: https://core.trac.wordpress.org/ticket/64112

[cbravobernal]

In 60957:

Block Bindings: Migrate Add link field link source key to field.

[60955] moved post-data key args to a better fit field one. Adds a forgotten in that changeset link to that field check.

Props bernhard-reiter.
Fixes #64112.

Committed in https://core.trac.wordpress.org/changeset/60957

Adds new field argument for term-data source.

Trac ticket: https://core.trac.wordpress.org/ticket/64112

[cbravobernal]

In 60958:

Block Bindings: Migrate term-data from key args to field.

Adds new field argument for term-data source.

Props bernhard-reiter.
Fixes #64112.