Add java.lang.Class to known immutables (#1695)

* Add java.lang.Class to known immutables

Storing a reference to a Class object triggers EI_EXPOSE_REP2, which is
not really fair, as java.lang.Class is really immutable.

Signed-off-by: Robert Varga <robert.varga@pantheon.tech>

* docs: update CHANGELOG

Co-authored-by: Kengo TODA <skypencil+github@gmail.com>

CHANGELOG.md

@@ -5,6 +5,8 @@ This is the changelog for SpotBugs. This follows [Keep a Changelog v1.0.0](http:
 Currently the versioning policy of this project follows [Semantic Versioning v2.0.0](http://semver.org/spec/v2.0.0.html).
 
 ## Unreleased - 2021-??-??
+### Fixed
+- Fixed immutable java.lang.Class as being flagged as EI ([#1695](https://github.com/spotbugs/spotbugs/pull/1695))
 
 ## 4.4.1 - 2021-09-07
 ### Changed

spotbugs/src/main/java/edu/umd/cs/findbugs/util/MutableClasses.java

@@ -17,7 +17,7 @@ public class MutableClasses {
     private static final Set<String> KNOWN_IMMUTABLE_CLASSES = new HashSet<>(Arrays.asList(
             "java.lang.String", "java.lang.Integer", "java.lang.Byte", "java.lang.Character",
             "java.lang.Short", "java.lang.Boolean", "java.lang.Long", "java.lang.Double",
-            "java.lang.Float", "java.lang.StackTraceElement", "java.math.BigInteger",
+            "java.lang.Float", "java.lang.StackTraceElement", "java.lang.Class", "java.math.BigInteger",
             "java.math.Decimal", "java.io.File", "java.awt.Font", "java.awt.BasicStroke",
             "java.awt.Color", "java.awt.GradientPaint", "java.awt.LinearGradientPaint",
             "java.awt.RadialGradientPaint", "java.Cursor.", "java.util.UUID", "java.net.URL",