Use un-escaped regexp literal for string matching.

[jeschkies]

What this PR does / why we need it:
The Regexp simplification would use the original string value instead of the escaped literal. E.g. | logfmt | level =~ `foo\.bar` would match the label level against foo\.bar and not foo.bar.

That is why the un-escaped literalt should be used in the equal and not-equal matchers.

Which issue(s) this PR fixes:
Fixes #9111

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
• Changes that require user attention or interaction to upgrade are documented in docs/sources/upgrading/_index.md

[grafanabot]

Hello @jeschkies!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If the pull request modifies CI behaviour, please add the type/ci label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[MasslessParticle]

Nice find!

[grafanabot]

The backport to release-2.8.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-9252-to-release-2.8.x origin/release-2.8.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 7109c56bdccc7c90e3f1fcdb16c8a586b3a773c6
# Push it to GitHub
git push --set-upstream origin backport-9252-to-release-2.8.x
git switch main
# Remove the local backport branch
git branch -D backport-9252-to-release-2.8.x

Then, create a pull request where the base branch is release-2.8.x and the compare/head branch is backport-9252-to-release-2.8.x.