Revert upgrade to rules_java 8.3.1.

[zhangskz]

This reverts protobuf's rules_java dependency back to 7.12.2 to remove the need for existing Bazel 6 / 7 WORKSPACE users to add additional loads for the 29.x minor release.

load("@rules_java//java:repositories.bzl", "rules_java_dependencies", "rules_java_toolchains")

rules_java_dependencies()
rules_java_toolchains()

This is a partial roll-back of fb8ee79.

Note that rules_java will likely still be upgrade in the 30.x release instead.