[SRU] MAAS 2.3.7

[Adam Collard]

[Impact]
This is a new upstream release that addresses various issues:

MAAS 2.3.7 includes fixes for the following issues:

 * LP: #1768870
 * LP: #1789932
 * LP: #1789721
 * LP: #1779980
 * LP: #1792401
 * LP: #1799161
 * LP: #1788780
 * LP: #1830365

MAAS 2.3.7

[Test Case]
MAAS testing has been done in various cases. This include:

1. Manual Fresh installation of MAAS
2. TODO: Manual upgrade from the previous Ubuntu Release.
3. Automated (CI) testing of MAAS install and operation as per the MAAS' CI.
4. Automated (CI) testing of MAAS install and operation against other Canonical's product (juju, Canonical OpenStack & Kubernetes) provided by the Canonical Solutions QA Team.
5. TODO: Manual split region/rack test are performed. This is to ensure that if we upgrade a MAAS Region to a newer version, the MAAS rack of the older version remains connected and operational.

All of this includes verifying normal operation, issues fixed, and ensuring that Canonical Cloud solutions can inter-operate. MAAS releases are also now vetted by the Solutions QA team.

[Regression Potential]
Minimal (For MAAS). MAAS is fully backwards compatible (the CI ensures that's the case) and handles upgrades from previous releases which result in the continuous operation of MAAS.