opentsdb: require Java 8 specifically

[apjanke]

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• Have you checked that there aren't other open pull requests for the same formula update/change?
• Have you built your formula locally with brew install --build-from-source <formula>, where <formula> is the name of the formula you're submitting?
• Does your build pass brew audit --strict <formula> (after doing brew install <formula>)? No, but it's not a regression.

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Addresses #19696 by requiring Java 8 exactly.

The audit failure:

$ brew audit --strict opentsdb
opentsdb:
  * `Use :optional` or `:recommended` instead of `depends_on HbaseLZORequirement if build.with?("lzo")`
Error: 1 problem in 1 formula
[✘ ~]
$

This was present before this PR, so it's not a regression. Looks like an edge case for that cop; I don't see how to fix it with :optional or :recommended here. Could fix this by just removing the --without-lzo option from hbase so it's always present, and no need to define a special requirement.

[ilovezfs]

test block failed

[apjanke]

I think I screwed up squashing commits when I pushed this initially.

Amended. Works locally for me.

[ilovezfs]

Thanks again @apjanke! 🏆