To reproduce bugs with recent Java and its compilers, it's nice to make this project ready to build with them.
Java 14 is the latest version, but 11 is the latest LTS so this PR uses Java 11 to build SpotBugs. refs #1244.
Gradle uses --release option to make sure the version of .class files are Java 8, so this change won't affect users.

At this moment we won't introduce module-info.java because some of our submodules provide the same package and it's not supported by JPMS.

Fix for RCN false positive

To pass the build with Java 11, this PR fixes #259 and #756 by ignoring RCN_REDUNDANT_NULLCHECK_OF_NONNULL_VALUE next to the end of try-block which catches java/lang/Throwable. See 7700cfc for detail.
This will fix reported false positives, but introduce false negatives in the following case, because we have no way to judge the bytecode to handle exception is generated by javac or not.

@NonNull
SomethingClosable s = ...;
try {
  ...
} catch (Throwable t) { // when the catch clause catches Throwable instance,
  // and when we have null check at the beginning of catch block,
  if (s != null) { // this is redundant null check but cannot be detected by SpotBugs any more
    ...
  }
}

I hope this false negative is acceptable to us. The redundant null check itself has less impact on runtime, I believe.