Online DDL: use shorter artifact table name in vitess migrations

[shlomi-noach]

Description

Fixes #14570
Follows up (and extends) on #14571

vitess migrations now use a shorter (by 2 characters) form of artifact table name. Instead of _*_vrepl it's _*_vrp. The change is minimal and everything should just continue to work as it was, with existing tests.

Related Issue(s)

• OnlineDDL: standard vitess artifact table name leaves no room for default _ibfk_... suffix #14570

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[shlomi-noach]

On hold while we debate #14582

[shlomi-noach]

Closing, see #14613 and #14582. Actuall change of behavior will only take place in v20. Specifically for Online DDL, there is no strict backwards compatibility issue, as table name is just written as vreplication rule/filter, and ithe table name's format is immaterial to any process. But to keep things in good order we will only apply the new naming together with TableGC etc.