Update assetPipelineVersion to v4 (#79)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

gradle.properties

@@ -5,7 +5,7 @@ casClientCoreVersion=3.6.4
 springSecurityCoreVersion=5.3.0
 springSecurityVersion=5.8.9
 javaxServletApiVersion=4.0.1
-assetPipelineVersion=3.4.7
+assetPipelineVersion=4.4.0
 projectDesc=The CAS plugin adds CAS single sign-on support to a Grails application that uses Spring Security.
 projectUrl=https://github.com/grails/grails-spring-security-cas
 githubSlug=grails/grails-spring-security-cas