Update api.php?setTempUnit function to require authentication

[kiyell]

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What does this PR aim to accomplish?:

There was a lack of a check to the $auth variable. This allowed any unauthenticated user to change the CPU temperature unit displayed of the pi-hole application. Adding the check of this variable fixes this issue.

The absence of authentication of this function was deemed not a security risk but maintainers were open to a potential merge depending on the release of v6.

Report: https://github.com/pi-hole/web/security/advisories/GHSA-mffw-5jmg-9wjq

How does this PR accomplish the above?:

Added && $auth authorization check before using setTempUnit parameter to change temperature.

Link documentation PRs if any are needed to support this PR:

N/A

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