Drop Python 3.6 support

[arm4b]

Creating an issue to track for the future v3.9.0 release.

Python 3.6 is deprecated, brings lots of difficulties with updating to the latest pip dependencies and should be eventually dropped from StackStorm.

Related:

• Update Orquesta to v1.6.0 #6050
• Update gitpython (security) #6063
• Drop Ubuntu 18.04 support from packaging st2-packages#731
• Bump minimum python version to 3.8 and Upgrade pantsbuild to 2.17 #6118