Change dateformat for git-commit-id-maven-plugin

Old formmat: yyyy-MM-dd'T'HH:mm:ssZ, RFC 822
New format:  yyyy-MM-dd'T'HH:mm:ssXXX ISO 8601

Related to git-commit-id/git-commit-id-maven-plugin#674.

This change is required to make the times produced by the
git-commit-id-maven-plugin usable for Maven's reproducible builds,
see https://maven.apache.org/guides/mini/guide-reproducible-builds.html.

Timestamp for reproducible output archive entries must either formatted
as ISO 8601 or as an int representing seconds since the epoch.

Example usage might be

   <properties>
     <project.build.outputTimestamp>${git.commit.time}</project.build.outputTimestamp>
   </properties>

See gh-39606

spring-boot-project/spring-boot-starters/spring-boot-starter-parent/build.gradle

@@ -148,7 +148,7 @@ publishing.publications.withType(MavenPublication) {
 							}
 							configuration {
 								delegate.verbose('true')
-								delegate.dateFormat("yyyy-MM-dd'T'HH:mm:ssZ")
+								delegate.dateFormat("yyyy-MM-dd'T'HH:mm:ssXXX")
 								delegate.generateGitPropertiesFile('true')
 								delegate.generateGitPropertiesFilename('${project.build.outputDirectory}/git.properties')
 							}