Drop JSR305 support in favor of JSpecify

[philwebb]

We should see what breaks if we remove "com.google.code.findbugs:jsr305"

[dmytrodanilenkov]

in scope of this issue do you want just drop or replace jsr305?

[philwebb]

We're not totally sure yet. I've added the "pending design work" label to make it clearer that we're not ready to accept contributions yet for this one.