Remove old migration tests

What does this MR do and why?

This remove all tests under specs/migration for releases up to v15.5.9, so we leave tests only from the last 6 releases.

See #412081 (comment 1396676457) for more details.

This was generated with the following commands

$ git co v15.5.9-ee

$ ls spec/migrations/*_spec.rb > tmp/specs-migrations-v15.5.9-ee

$ git co master

$ git co -b ka/prune-migration-specs-v15.5.9-ee

$ xargs sh -c 'rm "$@"' < tmp/specs-migrations-v15.5.9-ee 

$ git add .

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.