fix: make secret scanning omitvalue case insensitive

[khendrikse]

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Summary

I've been trying to reproduce a problem where someone properly configured the SECRETS_SCAN_SMART_DETECTION_OMIT_VALUES env var, yet the value was still seen as a secret. I have not been able to reproduce it but I did notice that in our check we did not make things case-insensitive.

The reason I think this might be necessary is because things like webpack could possibly change the casing of values.

Right now this is the only solution I could think of, apart from adding an additional check if we find a secret, to double check if it is in the omit env var, before we decide to fail the build. I'd rather try this first.

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[github-actions]

This pull request adds or modifies JavaScript (.js, .cjs, .mjs) files.
Consider converting them to TypeScript.

[aitchiss]

Oh interesting! I guess this makes sense? There is a tiny part of me that feels like if I omit Ls0t123 then ls0t123 shouldn't be omitted... but I think on balance this is a good fix and worth trying 👍