Only use IPv6 in the webserver when available

[DL6ER]

What does this implement/fix?

Check that IPv6 support is not disabled either via the boot command line or at runtime (via /etc/sysctl.conf) before trying to launch the webserver for the first time. This will only set the ports once on the very first start of Pi-hole v6.0 and can be freely edited in /etc/pihole/pihole.toml (webserver.ports) to whatever the user deems appropriate.

Hint: This will typically disable binding to IPv6 in docker containers as they most often have net.ipv6.conf.all.disable_ipv6 = 1

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
• I signed off all commits. Pi-hole enforces the DCO for all contributions
• I signed all my commits. Pi-hole requires signatures to verify authorship
• I have read the above and my PR is ready for review.

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/start-of-webserver-failed-web-interface-will-not-be-available/65545/8