Bump Gradle to 8.14.3.

For this purpose, I temporarily resurrected [`update_gradle.sh`](http://google3/third_party/gradle/update_gradle.sh;rcl=775340930), and I left a couple notes in cl/761102714.

I somewhat expect for this upgrade to break someone after submission (when a system like Kokoro runs tests). (TGP is clean, but I doubt that it offers significant test coverage.) I'm happy to roll back if there are issues; I'd just like to _attempt_ an upgrade so that we get some idea of what kind of testing a future upgrade might need and so that I record the existence of the `update_gradle.sh` script for anyone who might need it in the future. (I may or may not undelete it after this CL.)

(If this CL does stick, it will be a step toward merging google/guava#7775.)

PiperOrigin-RevId: 785527343

Author: cpovirk