Bump alpine from 3.20.5 to 3.21.3 in /src/test/resources/org/jvnet/hudson/plugins/platformlabeler/alpine/3.20.5

Bump alpine from 3.20.5 to 3.21.3 in /src/test/resources/org/jvnet/hudson/plugins/platformlabeler/alpine/3.20.5 #888