Require JDK 17 as the minimum version to run NullAway

[msridhar]

Error Prone has required JDK 17 since Sept. 2024 (version 2.32.0). NullAway currently still runs on JDK 11. Requiring JDK 17 could let us do some cleanup:

1. Text blocks would ease writing of tests.
2. Eventually, better support for reading type-use annotations from bytecodes may be ported back to JDK 17 (https://bugs.openjdk.org/browse/JDK-8346471). If we could rely on this fix, we could delete a lot of messy and tricky code.
3. We could also use features like records and pattern matching instanceof to clean up the code more.

Nothing urgent, but we should figure out when to make this transition. Maybe Sept. 2025 would be a good time?

[msridhar]

We are past the originally-proposed September 2025 timeline. I propose we cut one more release that supports JDK 11, and then require JDK 17. Please comment if this would cause problems for you.