FindJava: Update check to avoid accepting macOS stub 'java' as Java

Since commit dd378258 we try to avoid using the macOS /usr/bin/java stub if no underlying implementation of Java is actually installed. However, the message that /usr/bin/java prints when there is no Java available has changed since then. Update our check to also look for the new message.

While at it, revise the way we suppress Java_JAVA_EXECUTABLE. Previously we set its cache entry to Java_JAVA_EXECUTABLE-NOTFOUND, but that would cause the same find-and-reject sequence to be followed every time CMake runs in a build tree. Instead, use the approach used by !4813 (merged) for svn. Leave the cache entry alone and just set a normal variable of the same name to hide it.

Backport: release