Fix case label removal

[JojoFlex1]

So like when removing labels using @rustbot label -E-needs-mcve, the removal fails for labels with capital letters. This is cause the comparison in remove_labels was case-sensitive so -e-needs-mcve wouldn't match E-needs-mcve on the issue.
Adding labels works fine because GitHub's API handles case-insensitivity on their end. But removing requires an exact match with the label name as it exists on GitHub.
Now
Changed filter and contains to filter_map + find with a case-insensitive comparison. This also ensures we use the actual label name from the issue in the DELETE request, not the user-typed version.
#2214

[rustbot]

Error: Label E-needs-mcve can only be set by Rust team members

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[apiraino]

@JojoFlex1 did you (or could you) test this change?

Self-assigning for review.

thanks

[JojoFlex1]

@apiraino at first i ran cargo build and cargo test relabel and all 7 tests passed. So now i added a unit test specifically for the case-insensitive matching in src/github/issue.rs.