Enforce stricter javadoc @SInCE rules

See gh-17360

Author: philwebb

spring-boot-parent/src/checkstyle/checkstyle-suppressions.xml

@@ -28,4 +28,5 @@
 	<suppress files="Rating\.java" checks="JavadocVariable" />
 	<suppress files="Direction\.java" checks="JavadocVariable" />
 	<suppress files="JooqExamples\.java" checks="AvoidStaticImport"/>
+	<suppress files="[\\/]spring-boot-docs[\\/]" checks="SpringJavadoc" message="\@since" />
 </suppressions>

spring-boot-parent/src/checkstyle/checkstyle.xml

@@ -7,5 +7,9 @@
 			<property name="illegalPkgs" value="sun, org.apache.commons.(?!codec|dbcp|dbcp2|logging|pool|pool2).*, com.google.common.(?!cache).*, org.flywaydb.core.internal"/>
 			<property name="regexp" value="true"/>
 		</module>
+		<module name="io.spring.javaformat.checkstyle.check.SpringJavadocCheck">
+			<property name="publicOnlySinceTags" value="true" />
+			<property name="requireSinceTag" value="true" />
+		</module>
 	</module>
 </module>