revert Errorprone to 2.28.0 where JDK 11 was still supported.

Author: kfriedberger

lib/ivy.xml

@@ -124,8 +124,8 @@ SPDX-License-Identifier: Apache-2.0
 
         <!-- Google error-prone
              Compiler adaptor with some useful checks for common errors. -->
-        <dependency org="com.google.errorprone" name="error_prone_core" rev="2.35.1" conf="build->default" />
-        <dependency org="com.google.errorprone" name="error_prone_annotations" rev="2.35.1" conf="build->default; contrib->sources" />
+        <dependency org="com.google.errorprone" name="error_prone_core" rev="2.28.0" conf="build->default" />
+        <dependency org="com.google.errorprone" name="error_prone_annotations" rev="2.28.0" conf="build->default; contrib->sources" />
 
         <!-- Eclipse JDT Compiler
              For additional compiler warnings. -->