Provide migrate plugin for metatag_summary_length variable

Bumping to the v2 branch.

This will also need to be reworked after 📌 Migrations: Metatag-D7 misc settings Needs review is committed.

damienmckenna → changed the visibility of the branch 3063088-migrate_summary_length to hidden.

This is postponed until ✨ Token to provide summary of text fields (D8) Needs work is finished.