CI: Update cloud configs, ci-man jobs to v3 agents

CI: Update cloud configs, ci-man jobs to v3 agents

This includes the following related changes:
* Update all agent configs to use v3 nodes (replacing v2-highcpu with
  v3-starter)
* Remove unused agents centos8-builder-1c-1g and ubuntu1804-2c-8g
* Rename the (previously unused) centos8-builder-2c-2g to be 2c-4g to
  properly reflect the system it is now using
* Switch ci-jobs and ci-packer jobs to the 2c-4g agents. These were
  previously set to use an undefined agent.

Issue-ID: LF-Jira IT-28322
Signed-off-by: Eric Ball <eball@linuxfoundation.org>
Change-Id: Ife2496060fee1ed985413afb3343818ebf70c5e2