Make string formatting consistent
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### Problem
Within `routes.go` (https://gitlab.com/gitlab-org/gitlab-workhorse/-/blob/master/internal/upstream/routes.go) and `upload_test.go` we're inconsistently using double quotes and back ticks to delimit strings.
## Proposal
Since back ticks and double quotes work differently in Go, but there doesn't appear to be inconsistently used for the same thing, I would propose identifying places where special characters are required and use double quotes there or, just standardise on double quotes.
## Examples
https://gitlab.com/gitlab-org/gitlab-workhorse/-/blob/master/internal/upstream/routes.go#L249
https://gitlab.com/gitlab-org/gitlab-workhorse/-/blob/master/upload_test.go#L138
/cc @steveazz
issue