ci: Remove Ubuntu 18.04 (and gcc 5 / 6)

Ubuntu 18.04 is EOL, so will not be supported in the next version of
Open MPI.  Remove from main's list of CI builders.  Ubuntu 18 was
the last distro still including GCC 5 and GCC 6, so remove them
as well.

Signed-off-by: Brian Barrett <[email protected]>

Author: bwbarrett

.ci/community-jenkins/Jenkinsfile

@@ -53,8 +53,8 @@ println('Tests Completed')
 // build stage is a map of different configurations to test.
 def prepare_check_stages() {
     def configure_options = ["--disable-dlopen", "--disable-oshmem", "--enable-builtin-atomic", "--enable-ipv6"]
-    def compilers = ["clang10", "gcc5", "gcc6", "gcc7", "gcc8", "gcc9", "gcc10"]
-    def platforms = ["amazon_linux_2", "amazon_linux_2-arm64", "rhel8", "ubuntu_18.04"]
+    def compilers = ["clang10", "gcc7", "gcc8", "gcc9", "gcc10"]
+    def platforms = ["amazon_linux_2", "amazon_linux_2-arm64", "rhel8"]
     def check_stages_list = []
 
     // Build everything stage