fix: improve log level discovery

[shantanualsi]

What this PR does / why we need it:

Currently detection of log levels is not truly case insensitive. For eg. "warn" or "WARN" is detected, however "Warn" is not. The implementation uses string.Contains over the entire log line to check for these levels.

However, checking ignoring case would need us to toLower the entire log line which is inefficient. This PR detects the level first -> "level": in case of JSON and level= in case of logfmt and then compares the next 12 characters after that ignoring case.

Which issue(s) this PR fixes:
Ref: #12645

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[icelynjennings]

For anyone who found themselves in this PR wondering if it's been merged, please see #12651