Rename JUnit reports to Unit Test Reports

What does this MR do?

Having the name of the feature be "JUnit test reports" can cause confusion over what type of support this feature has. While it requires the JUnit report XML format, many different languages support that format. For a non-GitLab user/team member this could lead away from investigating a feature that might work well for them, just because their primary use case does not involve Java.

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