Update FTL compiling docs

[DL6ER]

What does this implement/fix?

Update compiling documentation for FTL following the recent update in pi-hole/docker-base-images#120. Also remove development-v6 in favor of development - the former is no more.

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

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

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[DL6ER]

This can be merged at any time as FTL compiles smoothly both against the old and the new versions. The related docker-base-images PR will ensure we are using the most recent dependencies in the CI as well.

[pralor-bot]

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

https://discourse.pi-hole.net/t/error-compiling-mbedtls-while-following-pi-hole-docs/82836/6