Modernize Java compiler invocations.
The currently used Java version in the industry is Java 8 or newer, since 2022.
See <https://en.wikipedia.org/wiki/Java_version_history>. And on macOS 11..13,
I'm seeing the "error: Source option 6 is no longer supported. Use 8 or later."
* gettext-runtime/configure.ac: Search for a Java compiler with source and
target 1.8.
* gettext-tools/configure.ac: Search for a Java compiler with source 1.8.
* gettext-tools/examples/hello-java/configure.ac: Likewise.
* gettext-tools/examples/hello-java-awt/configure.ac: Likewise.
* gettext-tools/examples/hello-java-swing/configure.ac: Likewise.
