Add variable migration to update existing tags

[ ] Functional review done
[ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
[ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
[ ] Security review done
[ ] Wording review done
[ ] Code review done
[ ] Tests were added if useful/possible
[ ] Reviewed for breaking changes
[ ] Developer changelog updated if needed
[ ] Documentation added if needed
[ ] Existing documentation updated if needed

matomo-org / tag-manager