Since we build almost everything these days using RHEL8 or UBI8 (with a few RHEL7 exceptions), we should consider moving some workload over to RHEL8.3 nodes instead of RHEL7.7.

This would also solve a few problems, such as having to get artifacts from EPEL or Fedora (eg., jq and skopeo).

Common changes across ported Jenkinsfiles:

• create new .groovy file
  • implement same rules from old job in new groovy script
  • ensure groovy script works with more than one JOB_BRANCH = 2.6, 2.7, ...

• rename Jenkinsfile to lowercase jenkinsfile (casc doesn't like uppercase)
  • switch to installSkopeoFromContainer("")
  • rename keytab/token variables used by withCredentials blocks
  • move old-school git clones to util.cloneRepo()
  • move old-school yq installs to util.installYq()
  • move old-school rpm installs to util.installRPMs(...)
  • replace any refs to BUILD_TIMESTAMP with (new java.text.SimpleDateFormat("yyyy-MM-dd_HH-mm-ss")).format(new Date())

• update get-sources-jenkins.sh to use correct Jenkins URL & new sources file format (fingerprint checks won't work) - CRW-1454