.travis.yml: Upgrade the Ubuntu ppc64le to 22.04 (Jammy).

Seeing the recent successes on RubyCI ppc64le Ubuntu jammy servers, it's time
to upgrade Travis ppc64le to jammy.

http://rubyci.s3.amazonaws.com/ppc64le/ruby-master/recent.html

Upgrade the gcc version to the latest version 11.4.0 to align the gcc
version used in the RubyCI ppc64le server's gcc version.

.travis.yml

@@ -63,8 +63,7 @@ env:
   - &ppc64le-linux
     name: ppc64le-linux
     arch: ppc64le
-    dist: focal
-    compiler: gcc
+    <<: *gcc-11
 
   - &s390x-linux
     name: s390x-linux