Try to restore broken pihole.toml from rotated files

[DL6ER]

What does this implement/fix?

If reading pihole.toml fails during startup of FTL, we should restore from the last known-to-be-good rotated config file in /etc/pihole/config_backups and only if they don't exist (migrating from v5.x -> v6.0), we start importing the legacy files.

Note that this problem is actually not an issue when the config file is modified and becomes invalid while FTL is running. In this case, FTL simply discards the file and overwrites it with its current configuration (which is in this case the last-known-to-be-good one).

This fixes an observation by @yubiuser where incorrectly escaped Umlauts triggered a "partial config reset". This was actually not a partial reset but FTL trying to restore from your previous v5.x files. This is fixed in this PR be instead using /etc/pihole/config_backup/pihole.toml.X (X = 1,2,3) to recreate /etc/pihole/pihole.toml.

---

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

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

---

By submitting this pull request, I confirm the following:

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/web-server-issues/66155/8

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[github-actions]

Conflicts have been resolved.

[yubiuser]

I think even more can go

[yubiuser]

Looks fine.

2023-11-22 13:54:33.490 [27128M] INFO: Config file /etc/pihole/pihole.toml not available: No such file or directory
2023-11-22 13:54:33.490 [27128M] INFO: Config backup file /etc/pihole/config_backups/pihole.toml.1 not available: No such file or directory
2023-11-22 13:54:33.490 [27128M] INFO: Using config backup /etc/pihole/config_backups/pihole.toml.2

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[github-actions]

Conflicts have been resolved.