Noop'ed the post_revisions.custom_excerpt population migration

[kevinansfield]

no issue

• the query can take a very long time to run on large sites causing problems during the upgrade process
• impact from not populating the column:
  • only has an effect when the inline excerpt beta is enabled
  • when beta enabled, if a revision created before the upgrade is restored then the excerpt will be removed (will be visibly empty in preview before restoring, if any edit has occurred on the post after upgrading then it can still be recovered by restoring the later version or copy/pasting from the preview)

[github-actions]

It looks like this PR contains a migration 👀
Here's the checklist for reviewing migrations:

General requirements

• Satisfies idempotency requirement (both up() and down())
• Does not reference models
• Filename is in the correct format (and correctly ordered)
• Targets the next minor version
• All code paths have appropriate log messages
• Uses the correct utils
• Contains a minimal changeset
• Does not mix DDL/DML operations

Schema changes

• Both schema change and related migration have been implemented
• For index changes: has been performance tested for large tables
• For new tables/columns: fields use the appropriate predefined field lengths
• For new tables/columns: field names follow the appropriate conventions
• Does not drop a non-alpha table outside of a major version

Data changes

• Mass updates/inserts are batched appropriately
• Does not loop over large tables/datasets
• Defends against missing or invalid data
• For settings updates: follows the appropriate guidelines