Allow java 15 and 17 for jvm_target

[andrewhamon]

This seems to fix #711

I've tested some Kotlin targets at my company that depend on java deps and I can successfully build with java 17, but haven't done anything more thorough than that.

Also, what do y'all think about removing the check altogether? Kotlin compiler will fail if an invalid version is provided anyways, and this Bazel-level check doesn't even spare users from that error if they try to use java 17 with a Kotlin compiler that is too old.

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[Bencodes]

Also, what do y'all think aout removing the check altogether?

This would be a better question for @restingbull

[Bencodes]

@andrewhamon can you also update the ## Custom toolchain section in the root README.md to list the newly supported jvm targets?

[andrewhamon]

@Bencodes just circled back to update the readme only to find you did it for me... thanks!