Don't accept files that are bigger than the upload limit when uploading plugins #16849

Description:

fixes #16847

Review

• [x] Functional review done
• [x] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [x] Security review done see checklist
• [x] Code review done
• [ ] Tests were added if useful/possible
• [x] Reviewed for breaking changes
• [x] Developer changelog updated if needed
• [x] Documentation added if needed
• [x] Existing documentation updated if needed

Noticed if a strange value is set in php.ini, it is used as is and shows up in data-max-size. Eg, if I use "8 M", data-max-size is "8 ". Setting it to blahblah or 8blahblahM sets data-max-size to "" which I think is correct, so it's just if there's a bunch of spaces and no math is done I guess.

@diosmosis I've improved the code a bit more and added a simple test