Accept both uppercase and lowercase for data_category in metric definition

What does this MR do?

Related to #334704 (closed).

This relaxes the metric definition JSON Schema constraint to allow both uppercase and lowercase data_category values, and handle lowercase values by default.

Does this MR meet the acceptance criteria?

Conformity

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• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
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• 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.