[Merge] ~chad.smith/ubuntu-release-upgrader:esm-support-pro-upgrades into ubuntu-release-upgrader:ubuntu/master

[Chad Smith]

The proposal to merge ~chad.smith/ubuntu-release-upgrader:esm-support-pro-upgrades into ubuntu-release-upgrader:ubuntu/master has been updated.

Description changed to:

Remaining barriers to upgrade support for Ubuntu Advantage. I've filed/ammended the current bugs for this feature/bugfix:

 [1] Add UA PPAs to mirrors.cfg: https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1893717

 [2] Drop apt cache lock inside runPostInstallScripts: https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upgrader/+bug/1897778


Commit details:
 - 3c23ab91. add ubuntu-advantage PPA to valid commercial mirror URLs

 - 6b6f79583. release apt cache lock during PostUpgradeScripts runs (allows ua tools to upgrade apt sources due to release contract differences)

For more details, see:
https://code.launchpad.net/~chad.smith/ubuntu-release-upgrader/+git/ubuntu-release-upgrader/+merge/391183
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