v0.5 backport: Restore SDK and Ingestion compatibility with Java 8 runtimes

[ches]

What this PR does / why we need it:

Backports #722 to v0.5.

Does this PR introduce a user-facing change?:

- Restores Feast Ingestion compatibility with Java 8 runtimes
- Restores Feast Java SDK compatibility with Java 8 runtimes

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[ches]

Hmm probably we also need to backport #725, and perhaps #728.

[ches]

/retest

Maven Central connection blip it seems

[woop]

Those tests dont add any value here I think. They are testing the current master branch. I've removed them from all non-master branches and all PRs. So they only trigger on a push to master and we can track them through the front page badge.

Wait a minute, they shouldn't have run on a PR in the first place. Strange.

[ches]

Wait a minute, they shouldn't have run on a PR in the first place. Strange.

I'd guess it's that b8456c1 isn't on v0.5-branch—my interpretation of this is that a pull request event is probably associated with the target branch as its Git ref, so the workflow configurations from the branch head will apply.