Removed deprecated __unstableMigrate function

[sbappan]

Description

This PR removes the deprecated __unstableMigrate function code and resolves #24931

How has this been tested?

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[youknowriad]

@noisysocks I believe you're one of the last persons who touched this, is it safe to remove now?

[noisysocks]

Looks good. We'll need to remove the code from Core that calls __unstableMigrate. Can you open a ticket on Trac with a patch that does this, @sbappan?

[youknowriad]

So it seems if we merge this PR, The plugin won't support 5.4 and 5.5, am I right? So maybe we should add a comment instead of removing.

[noisysocks]

So it seems if we merge this PR, The plugin won't support 5.4 and 5.5, am I right?

Ah, correct. We can't remove this code until our Requires at least is 5.6. We should proceed with opening and committing a Trac ticket that removes three this is called right now though.

[Mamaduka]

The plugin now requires WP 5.6 or higher.

[t-hamano]

The __unstableMigrate function appears to have been properly removed by #39795.
Should we close this PR?

[Mamaduka]

@t-hamano, you're right. We can close this PR.