[SG-35514] logging(gitstart): migrate gitserver logging

Created by: gitstart-sourcegraph

Description

The main existing logger usage to replace are calls to log15.

Test plan

All ~125 occurrences of log15 calls in files that appear related to gitserver are replaced with logging calls to an appropriately scoped log.Logger instance, e.g. ones held by the parent struct:

https://sourcegraph.com/search?q=context:global+r:%5Egithub%5C.com/sourcegraph/sourcegraph%24++content:%22log15.%22+lang:go+f:gitserver&patternType=literal

Green CI builds.

Refs

• SourceGraph Issue
• GitStart ticket