Upgrade Error Prone fork 2.36.0-picnic-1 -> 2.36.0-picnic-2 (#1499)

This new release is published using GitHub Packages rather than
JitPack. This comes with a simpler release setup and a (hopefully) more
stable package repository. A minor downside is that GitHub Packages
require authenticated access, even for read access. The documentation
has been updated accordingly.

The new release is no longer published using a custom groupId, enabling
some build simplifications.

While there, some obsolete fork-related documentation and configuration
is dropped.

See:
- https://github.com/PicnicSupermarket/error-prone/releases/tag/v2.36.0-picnic-2
- https://github.com/PicnicSupermarket/error-prone/compare/v2.36.0-picnic-1...v2.36.0-picnic-2

website/generate-version-compatibility-overview.sh

@@ -86,7 +86,7 @@ for eps_version in ${eps_versions}; do
         -Ppatch \
         -Pself-check \
         -Dverification.skip \
-        -Dversion.error-prone-orig="${ep_version}" \
+        -Dversion.error-prone="${ep_version}" \
       && echo "SUCCESS: { \"eps_version\": \"${eps_version}\", \"ep_version\": \"${ep_version}\" }" || true
     # Undo any changes applied by the checks.
     git checkout -- '*.java'