covert getPluginUmdChunks from private to protected #19212

Description:

Fixes: #19207
covert getPluginUmdChunks from private to protected.

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

Sorry @peterhashair I meant to do this on the next_release branch which means you have some 4.x-dev changes here that shouldn't be. Can you maybe redo this on the next_release branch (maybe easier than fixing this one).

@justinvelluppillai sorry, my bad.

@justinvelluppillai I look the failed tests, 4 are random, 1 is about SEO ranking. all unrelated.