Speed up checking whether fields are markdown fields or not

What does this MR do?

Extracted from !63040 (merged)

Gitlab::MarkdownCache::FieldData wraps a cache, and we consult cache key membership in a few places. Currently, we do that by effectively calling @data.keys.include?

Calling @data.key? is a tiny bit more efficient, and reads more cleanly in the places it's used.

No changelog since I don't think this is going to be a noticeable performance change.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
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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.)
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