refactor: replace experimental maps and slices with stdlib

[Juneezee]

What this PR does / why we need it:

The experimental functions are now available in the standard library since Go 1.23 ¹.

The only difference is that maps.Keys and maps.Values in the standard library return an iterator, while maps.Keys and maps.Values in the golang.org/x/exp package return a slice.

Which issue(s) this PR fixes:

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
• 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

Footnotes

1. https://go.dev/doc/go1.23#new-unique-package ↩

[cyriltovena]

Thanks LGTM