Make unique index compatible with database without long key support #18723

Description:

Not sure if that is the most elegant solution. But can't think of something better right now.

fixes #18721

Review

• [ ] 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

This pull request has been mentioned on Matomo forums. There might be relevant details there:

https://forum.matomo.org/t/4-7-0-b2-update-fails-with-mysql-5-5/44603/10

This pull request has been mentioned on Matomo forums. There might be relevant details there:

https://forum.matomo.org/t/does-4-71-fix-the-fatal-sql-error-on-upgrade-using-mysql-version-5-5/44656/2