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

refaster-compiler/pom.xml

@@ -15,19 +15,6 @@
     <url>https://error-prone.picnic.tech</url>
 
     <dependencies>
-        <dependency>
-            <groupId>${groupId.error-prone}</groupId>
-            <artifactId>error_prone_annotations</artifactId>
-            <scope>provided</scope>
-        </dependency>
-        <dependency>
-            <groupId>${groupId.error-prone}</groupId>
-            <artifactId>error_prone_check_api</artifactId>
-        </dependency>
-        <dependency>
-            <groupId>${groupId.error-prone}</groupId>
-            <artifactId>error_prone_core</artifactId>
-        </dependency>
         <dependency>
             <groupId>${project.groupId}</groupId>
             <artifactId>refaster-support</artifactId>
@@ -37,6 +24,19 @@
             <artifactId>auto-service-annotations</artifactId>
             <scope>provided</scope>
         </dependency>
+        <dependency>
+            <groupId>com.google.errorprone</groupId>
+            <artifactId>error_prone_annotations</artifactId>
+            <scope>provided</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.google.errorprone</groupId>
+            <artifactId>error_prone_check_api</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>com.google.errorprone</groupId>
+            <artifactId>error_prone_core</artifactId>
+        </dependency>
         <dependency>
             <groupId>com.google.guava</groupId>
             <artifactId>guava</artifactId>