Bump alpine from 3.18.12 to 3.21.3 in /src/test/resources/org/jvnet/hudson/plugins/platformlabeler/alpine/3.18.12

Bump alpine from 3.18.12 to 3.21.3 in /src/test/resources/org/jvnet/hudson/plugins/platformlabeler/alpine/3.18.12 #1354