update-notifier: adopt renamed esm infra and apps pockets

[Chad Smith]

Placeholder bug for discussion with online-services and amongst the ubuntu server team about viability of this solution.

do-release-upgrade only supports apt suite names confirming to the format:
<release>-(security|upgrades|proposed)

This begs that ESM apt repos are named something like:
xenial-updates
bionic-security
etc.

instead of:
bionic-infra-security/
bionic-infra-updates/
focal-infra-security/
focal-infra-updates/

Since update-notifier reports, via motd, the upgradable and security packages available in ESM-* repos we need to cope with this apt suite rename when it happens.

This fix will only land in groovy and get SRU'd to Xenial, Bionic and Focal once the contract services can support a the standard <release>-(security|updates) suite names.

[Chad Smith (chad.smith)]

Rejecting this bug. do-release-upgrade can be extended to support <release>-infra-(updates|security) custom APT suites. So no need to reshuffle custom esm apt suite names (again).