Skip multi-version upgrade migration spec on default branches

Nailia Iskhakovarequested to merge
500212-skip-multi-version-upgrade-migration-spec-def-branches into master

What does this MR do and why?

Skip multi-version upgrade migration spec on default branches. This will allow to ensure that the job doesn't run in older backport releases as it happened in #500212 (closed). There is no need to run the job in default branches, as it's required to pass and will be pass on master. While for older stable branches, the job should be skipped as it wouldn't work correctly against older versions due to automatically updated PG dump used in the test, see context gitlab-org/quality/quality-engineering/team-tasks#3028 (closed)

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