[pkg.jenkins.io,releases] Finish cleanup of mirrorbrain

[dduportal]

I thought that #2888 was finished, but #2967 clearly demonstrates that there are still some leftovers of mirrorbrains:

• User mirrorbrain still present on the machine and used by the releases
  • https://github.com/jenkins-infra/release/blob/1979e858fbaaacaaf54ed10122d11dbc0ba79d93/utils/release.bash#L610
  • https://github.com/jenkins-infra/release/blob/6669feed8e31482be18cb1b3516bdd87ec4f6d06/profile.d/experimental#L19
  • Fix the runbook https://github.com/jenkins-infra/runbooks/blob/e36ae6588fd03b3a2220f620a7db4fd1f49efe69/security/README.adoc#L55
  • Manage blobxfer installation (and any dependency of the mirror scripts)
• Put back into Puppet management the mirror-scripts
• Correct comment at https://github.com/jenkins-infra/jenkins-infra/blob/65308dd71c7f17bbb1c96608647bfb97bdcdff64/dist/role/manifests/pkg.pp#L2
• Fix the public documentation: https://github.com/jenkins-infra/jenkins.io/blob/002ad0b3450822ffe7b68456927f678b4125db91/content/projects/infrastructure/index.adoc#L111

Closes #505

[dduportal]

We have #505 that we want to revert as part of this cleanup

[dduportal]

Stuff to NOT forget to migrat from the former mirrobrain.pp profile + from the production VM (and that has been managed manually)

• Downloads /latest directory out of date #3034 (comment)
• Downloads /latest directory out of date #3034 (comment)

[dduportal]

While working on #3414 we realized the following:

• The user mirrorbrain is too much coupled with the whole pkg VM (pkg.origin.jenkins.io and updates.jenkins.io). It was used for multiple purposes: the former "mirrors.jenkins.io" (mirrorbrain) service, but also by all the mirrors rsyncs as per [INFRA-657] Migrate update-site rsyncing jobs from www-data to mirrorbrain #505 and Check if we could replace blobxfer by azcopy #3414 (comment)

As such, we decided to bring back the mirrorbrain user under puppet management in jenkins-infra/jenkins-infra#3357 as we keep using it.
Note that it implied only a minimalist set of resources: the user itself, only one of its crontabs (used for full sync), current SSH keys for archives.j and OSUOSL servers.

Last mile before closing this issue: jenkins-infra/jenkins-infra#3352

[dduportal]

With jenkins-infra/jenkins-infra#3352 completed we can close this issue