(PA-6881) Configure a default gem_uninstall command

[donoghuc]

With the addition logic in #901 for deduplicating rexml gems an inadvertant requirement was imposed on projects to define a gem_uninstall command. This command should be ubiquitous (especially with impending ruby 3 only streams) so instead of requiring all projects to configure it a default is added. This setting is still configurable at a project level, but is not required.

[donoghuc]

Without this all the runtime projects that use rexml break as they previously were not required to have a gem_uninstall command configured.

https://jenkins-platform.delivery.puppetlabs.net/view/Skeletor%20CI%20Goalie/job/platform_pe-bolt-server-runtime_runtime-vanagon-packaging_main/427/
https://jenkins-platform.delivery.puppetlabs.net/view/puppet-runtime/view/pdk-runtime/job/platform_pdk-runtime_runtime-vanagon-packaging_main/467/
https://jenkins-platform.delivery.puppetlabs.net/view/puppet-runtime/view/pe-installer-runtime/job/platform_pe-installer-runtime_runtime-vanagon-packaging_main/382/

[donoghuc]

Build a bolt-server runtime just to double check:

(base) ➜  puppet-runtime git:(default-gem_uninstall) ✗ bundle exec vanagon build pe-bolt-server-runtime-main sles-15-x86_64
...
cp pe-bolt-server-runtime-main-202409030.1.gdedd733.sles-15-x86_64.tar.gz output
sha1sum output/pe-bolt-server-runtime-main-202409030.1.gdedd733.sles-15-x86_64.tar.gz > output/pe-bolt-server-runtime-main-202409030.1.gdedd733.sles-15-x86_64.tar.gz.sha1
Warning: Permanently added 'eared-principle.delivery.puppetlabs.net' (ED25519) to the list of known hosts.

Installs cleanly.