Drop Java 7 compatibility

[cpovirk]

We claim compatibility with it, and we might well provide it as recently as our latest release, but we haven't been testing for compatibility since f730377 (since we didn't do whatever the Maven equivalent of https://jakewharton.com/build-on-latest-java-test-through-lowest-java/ is—I thought I had a Maven link handy, but I didn't find it instantly, so I gave up):

jimfs/.github/workflows/ci.yml

Line 17 in 8428991

java: [ 15, 11, 8 ]

I believe that we lost compatibility as of the ICU4J upgrade that we merged a while back (but haven't put in a release yet):

• c6a44ad
• unicode-org/icu@9acba58

So mainly I think we'll just want to update our docs whenever we make another release.

[cpovirk]

RE: Maven Toolchains: google/error-prone#3895 (comment)

[cpovirk]

We added a direct use of a Java 8 API in bdcd240, as I just noticed in #303.

I'm not sure if I had a reason to avoid setting up Animal Sniffer back then or even back in #113 (though I added other enforcement then). It might be a good idea to do.