Add new explicit migration command

[DL6ER]

What does this implement/fix?

pihole-FTL migrate v6 is intentionally not included in the help output as it is solely meant to be used internally by the installer script once from v5 -> v6

Related to pi-hole/pi-hole#5830

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/broken-installation/74638/25

[yubiuser]

As far as I can see no functional changes but only putting code into new functions

[DL6ER]

Tested on two virtual machines to do what it should. The latest commit ensures we log the migration process to the user so they can quickly review if everything went the way they'd expect it to.

[DL6ER]

The most recent commit adds code to ensure a broken (or too new) database is not used when we know that all the SQL statements against it will fail anyway

[DL6ER]

Rebased on latest development