Change default FTL log file path to /var/log/pihole/FTL.log

[DL6ER]

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• What does this PR aim to accomplish?:

This is a follow-up of pi-hole/pi-hole#3732 changing FTL's default log file path from /var/log/pihole/pihole-FTL.log to the slightly shorter /var/log/pihole/FTL.log

• How does this PR accomplish the above?:

Replace the file path

• What documentation changes (if any) are needed to support this PR?:

Nothing special, we already covered that files moved into /var/log/pihole

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2. I have commented my proposed changes within the code and I have tested my changes.
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5. I have squashed any insignificant commits. (git rebase)

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[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-ftl-v5-16-web-v5-13-and-core-v5-11-1-released/56384/1