Make target Ghidra repo configurable

[encounter]

Primarily, this is intended to make it easier for forks to update the target Ghidra repository. An example of these changes: https://github.com/encounter/ghidra-ci

• Centralize repo and dependency version configuration
• Update all actions (+ replace unmaintained actions/create-release)
• Simplify artifact & release handling
• Better release artifact names (e.g. ghidra_11.1_DEV_20240120.zip instead of release.zip)
• Allow prevRev to be empty ("fallback" changelog is generated instead)
• Various cosmetic changes (names for all steps, etc)

Thanks for setting this up! It's saved an immense amount of time.

[roblabla]

Hey, thanks for the interest!

Most of the changes look good, but I'll need to merge them piece by piece to make sure I understand what the changes entail.

This repository's CI is a bit sensitive since it contains my Apple Dev Certificate and API keys to sign/notarize the releases, so this may take a bit.

[roblabla]

Sorry for the very long delay.

I pulled some of your changes manually, split them into dedicated commits, and adapted them a bit. On the externally visible side of things:

• The changelog now uses the fallback when prevRev isn't available
• The PAT_TOKEN is now unnecessary for the workflow dispatch call.
• It's possible to configure the ghidra repo by setting a variable (GHIDRA_REPO) in the repo settings (settings -> secrets and variables -> actions -> variables -> new repository variable). I added some documentations in the README on how to achieve this.

I also included a few of your other changes like putting the JAVA_VERSION and GRADLE_VERSION in an env block so it can be easily changed, renaming ghidra-ci-roblabla to ghidra-ci, and some of the simplifications.

A lot of the remaining changes are either cosmetic in nature (changing the step names, yaml formatting and whatnot), or not really necessary (upgrading the actions) so I left them out for now. I'll try to go over them soon-ish.

I also didn't pull the change to the artifact name, as having a consistent artifact name is convenient when trying to wget the latest version easily ^^'.

Thanks for the contribution.