Remove support for older distros

[adarshan-intel]

Description of the Changes

This pull request removes support for older distributions from the GSC config.yaml.template and associated documentation. The following distributions have been removed:

• Ubuntu: 21.04, 20.04, 23.04
• Debian: 10, 11
• CentOS: 8

Key Changes:

• Documentation/index.rst: Updated to reflect the current supported distributions.
• config.yaml.template: Removed references to the unsupported distributions.
• test/README.rst: Modified Docker build and test commands to utilize Ubuntu 22.04.

How to Test this PR

To verify the changes:

1. Ensure the GSC build and deployment processes function correctly with the updated list of supported distributions.
2. Follow the updated instructions in the test/README.rst.

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This change is 

[adarshan-intel]

@mkow @woju Can you review?

[adarshan-intel]

@woju @mkow Please review