feat: remove target lag and keep just maximum lag

[grobinson-grafana]

What this PR does / why we need it:

This commit is an experiment to see if we can remove target lag and just keep maximum lag. The goal here is to simplify the code and make the catch up feature easier to reason about. However, if we find that this causes stale reads, we can add it back.

Which issue(s) this PR fixes:
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

[cyriltovena]

LGTM