Promtail: Fix timestamp parser for short year format

[Falco20019]

What this PR does / why we need it:
Using a short year format for timestamp in the pipeline break if trying to parse older log files.

...
scrape_configs:
- pipeline_stages:
...
  - timestamp:
      source: time
      format: 02.01.06 03:04:05.000
...

Which issue(s) this PR fixes:
Fixes #2653

Special notes for your reviewer:
I don't have a golang environment on my machine and was not able to properly run the Makefile on windows either. Only traced the cause by looking into the code base. I hope there is CI testing it.

Checklist

• Documentation added
  -- Not necessary as it's a bug fix
• Tests updated

[CLAassistant]

All committers have signed the CLA.

[cyriltovena]

LGTM