Add checkstyle rule for toLowerCase/toUpperCase

This commit adds a checkstyle rule that prevents the re-introduction of
such calls.

See gh-33708

src/checkstyle/checkstyle-suppressions.xml

@@ -12,6 +12,8 @@
 
 	<!-- Global: tests and test fixtures -->
 	<suppress files="[\\/]src[\\/](test|testFixtures)[\\/](java|java21)[\\/]" checks="AnnotationLocation|AnnotationUseStyle|AtclauseOrder|AvoidNestedBlocks|FinalClass|HideUtilityClassConstructor|InnerTypeLast|JavadocStyle|JavadocType|JavadocVariable|LeftCurly|MultipleVariableDeclarations|NeedBraces|OneTopLevelClass|OuterTypeFilename|RequireThis|SpringCatch|SpringJavadoc|SpringNoThis"/>
+	<suppress files="[\\/]src[\\/](test|testFixtures)[\\/](java|java21)[\\/]" checks="RegexpSinglelineJava" id="toLowerCaseWithoutLocale"/>
+	<suppress files="[\\/]src[\\/](test|testFixtures)[\\/](java|java21)[\\/]" checks="RegexpSinglelineJava" id="toUpperCaseWithoutLocale"/>
 	<suppress files="[\\/]src[\\/](test|testFixtures)[\\/](java|java21)[\\/]org[\\/]springframework[\\/].+(Tests|Suite)" checks="IllegalImport" id="bannedJUnitJupiterImports"/>
 	<suppress files="[\\/]src[\\/](test|testFixtures)[\\/](java|java21)[\\/]" checks="SpringJUnit5" message="should not be public"/>
 

src/checkstyle/checkstyle.xml

@@ -221,6 +221,22 @@
 			<property name="message" value="Please use class literals for primitives and void -- for example, int.class instead of Integer.TYPE."/>
 			<property name="ignoreComments" value="true"/>
 		</module>
+		<module name="com.puppycrawl.tools.checkstyle.checks.regexp.RegexpSinglelineJavaCheck">
+			<property name="id" value="toLowerCaseWithoutLocale"/>
+			<property name="format" value="\.toLowerCase\(\)"/>
+			<property name="maximum" value="0"/>
+			<property name="message"
+					  value="String.toLowerCase() should be String.toLowerCase(Locale.ROOT)"/>
+			<property name="ignoreComments" value="true"/>
+		</module>
+		<module name="com.puppycrawl.tools.checkstyle.checks.regexp.RegexpSinglelineJavaCheck">
+			<property name="id" value="toUpperCaseWithoutLocale"/>
+			<property name="format" value="\.toUpperCase\(\)"/>
+			<property name="maximum" value="0"/>
+			<property name="message"
+					  value="String.toUpperCase() should be String.toUpperCase(Locale.ROOT)"/>
+			<property name="ignoreComments" value="true"/>
+		</module>
 		<module
 			name="com.puppycrawl.tools.checkstyle.checks.regexp.RegexpSinglelineJavaCheck">
 			<property name="maximum" value="0"/>