Convert metric definition data categories to lowercase

What does this MR do?

Related to #334704 (closed). Follow-up to !67469 (merged).

This converts data_category values to lowercase in metric definitions, since this is now allowed in the schema and should be the default.

This is a large MR (in the sense of a massive number of files modified), but all these files have the exact same change and it wouldn't make sense to divide this into smaller parts. The changes were generated by this sed one-liner:

sed -i -- 's/data_category: [A-Z]/\L&/g' config/metrics/**/*.yml ee/config/metrics/**/*.yml

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.