Remove Java 13 from runtime testing matrix

With Java 14 soon to be GA 13 will be EOL so there's no need to continue
testing on it.

Signed-off-by: Mark Vieira <portugee@gmail.com>

.ci/matrix-runtime-javas.yml

@@ -7,7 +7,6 @@
 
 ES_RUNTIME_JAVA:
   - java11
-  - openjdk13
   - openjdk14
   - openjdk15
   - zulu11