fix(secret): convert severity for custom rules

[DmitriyLewen]

Description

By default we filter all secrets by severity.
However, there may be cases where user uses wrong severity (wrong name or case (e.g. lower case)) for custom rules.
We need to convert these severity levels to upper case to avoid incorrect filtering.

example - #6454 (reply in thread)

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[knqyf263]

LGTM. Is there any GitHub Issue for this fix? Or did you stumble upon this problem and quickly fix it?

[DmitriyLewen]

I checked the user custom rules for #6454 and saw this issue - #6454 (reply in thread).

When I found reason for lost secrets, I quickly created this PR.

[knqyf263]

OK, thanks!