Misc fixes and cleanup

- Remove the `demo` project which isn't really useful.
  * Instead the `addons` subfolder is made top-level and that's where stuff gets written.
- Update the README following recent PRs (#191, #199) as OpenSSL is now compiled from source for all platforms.
  * Also remove obsolete `build_openssl_universal_macos.sh` script.
- Remove the bogus `plugin.cfg` which isn't necessary, and was confusing users who tried to enable it.
  * Fixes #267.
- Bump min SCons version to 3.1.2 and Python to 3.6.
- Bump `compatibility_minimum` to `4.2.0` following #196.
- Remove some code in `godot-git-plugin/SCsub` that seems redundant with `godot-cpp` config.
- Remove unnecessary `.exp` and `.lib` files in Windows artifact, rename its folder to `windows`.
- Remove `export-ignore`s in `.gitattributes`, they're incomplete and not actually doing anything usable.
- Fix artifacts URL handling in `release.sh`, make it executable.
- CI: Update clang-format check to version 18.

release.sh

@@ -5,19 +5,21 @@
 echo "Enter the new version number (e.g. 1.2.1):"
 read version
 
-echo "Enter the Windows x64 release ZIP URL:"
-read windowsZIPURL
+echo "Enter the Windows x64 release ZIP artifact ID or URL:"
+read windowsArtifactID
 
-echo "Enter the Linux x64 release ZIP URL:"
-read linuxZIPURL
+echo "Enter the Linux x64 release ZIP artifact ID or URL:"
+read linuxArtifactID
 
-echo "Enter the MacOS universal release ZIP URL:"
-read macZIPURL
+echo "Enter the MacOS universal release ZIP artifact ID or URL:"
+read macArtifactID
 
 # wget-ing the github.com URL gives a 404, so we use the method proposed here - https://github.com/actions/upload-artifact/issues/51#issuecomment-735989475
-windowsZIPURL=${windowsZIPURL/github.com/nightly.link}
-linuxZIPURL=${linuxZIPURL/github.com/nightly.link}
-macZIPURL=${macZIPURL/github.com/nightly.link}
+# The ${foo##*/} syntax extracts the string after the last '/' in case it's a full artifact URL.
+baseURL=https://nightly.link/godotengine/godot-git-plugin/actions/artifacts/
+windowsZIPURL=${baseURL}${windowsArtifactID##*/}.zip
+linuxZIPURL=${baseURL}${linuxArtifactID##*/}.zip
+macZIPURL=${baseURL}${macArtifactID##*/}.zip
 
 wget -O windows.zip $windowsZIPURL
 wget -O linux.zip $linuxZIPURL
@@ -43,7 +45,7 @@ sed -i "s/version=\"[^\"]*\"/version=\"v${version}\"/g" $pluginPath/plugin.cfg
 cp LICENSE $pluginPath/LICENSE
 cp THIRDPARTY.md $pluginPath/THIRDPARTY.md
 
-zip -r $releasePath.zip $addonsPath 
+zip -r $releasePath.zip $addonsPath
 
 rm -rf $releasePath
 rm -rf windows