Remove support for Python 3.7

[ptitzler]

Is your feature request related to a problem? Please describe.
Elyra currently requires Python version 3.7 or later for build and deployment. JupyterLab version 4.0 removes support for Python 3.7 and therefore we'll have to too.

Describe the solution you'd like

• Update README and documentation (bump all 3.7 references to 3.8)
• Update CI test matrix
• There should be no impact on on runtime images because the existing validation already rejects images that have this version installed

Additional context

• Official Python end of life: June 27, 2023
• This would probably be a good opportunity to add Python 3.11 to the server test matrix, if that version is supported by JupyterLab