Disable super_read_only during PRS for v15->v19 migration

[tanjinx]

In v19, PRS set the old primary super_read_only unconditionally during the demotion process. If we downgrade that tablet to v15, restart will fail with the error:

I1014 11:21:28.334578  336986 metadata_tables.go:82] Populating _vt.local_metadata table...
F1014 11:21:28.335299  336986 tm_init.go:775] RestoreFromBackup failed: The MySQL server is running with the --super-read-only option so it cannot execute this statement (errno 1290) (sqlstate HY000) during query: CREATE DATABASE IF NOT EXISTS _vt

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• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
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