[release-2.9.x] fix: frontend: Use `net.JoinHostPort` to support IPv6 addresses (#10650)

[periklis]

What this PR does / why we need it: Manual backport for #10650

Which issue(s) this PR fixes: Fixes #

Special notes for your reviewer:

Checklist

• [x] Reviewed the CONTRIBUTING.md guide (required)
• [ ] Documentation added
• [x] Tests updated
• [ ] CHANGELOG.md updated
  • [ ] If the change is worth mentioning in the release notes, add add-to-release-notes label
• [ ] Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• [ ] For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• [ ] 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

[periklis]

lgtm, I noticed no CHANGELOG update, maybe it makes sense adding a line there

I have to figure out how the correct labeling on GH is meant to be used due to the new release process.

[dbazhal]

Hi guys! Can this please be checked?) @chaudum

[chaudum]

@periklis Can you rebase the PR against release-2.9.x to trigger the new GH actions?

[periklis]

@chaudum Rebased! Can you take a look please