Bump github.com/prometheus/common from 0.20.0 to 0.21.0

Bumps [github.com/prometheus/common](https://github.com/prometheus/common) from 0.20.0 to 0.21.0.
- [Release notes](https://github.com/prometheus/common/releases)
- [Commits](prometheus/common@v0.20.0...v0.21.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

go.mod

@@ -6,5 +6,5 @@ require (
 	github.com/golangci/golangci-lint v1.39.0
 	github.com/nginxinc/nginx-plus-go-client v0.8.0
 	github.com/prometheus/client_golang v1.10.0
-	github.com/prometheus/common v0.20.0
+	github.com/prometheus/common v0.21.0
 )