Display warning if unsupported OS is detected

[PromoFaux]

By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes, and have included unit tests where possible.
• I am willing to help maintain this change if there are issues with it later.
• I give this submission freely and claim no ownership.
• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)

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What does this PR aim to accomplish?:

This adds an additional check to the top of the install script to determine whether or not the script is being invoked on a supported system.

Unsupported OS message:

Unsupported - but user acknowledged - message:

Supported OS Message:

As discussed in #3421

[dschaper]

Same changes here as on #3434

[pralor-bot]

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

https://discourse.pi-hole.net/t/pi-hole-5-1-released/35577/1

[pralor-bot]

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

https://discourse.pi-hole.net/t/unsupported-os-error-when-upgrading-to-5-1/35630/2