Add warning for dropping Ubuntu 14.04 (Trusty Tahir) and Debian 8 (Jessie) support

[jiasli]

Since Python 3.6 has reached its end-of-line, I am trying to bump embedded Python to 3.8 for deb packages (Azure/azure-cli#20869). During that, I found Python 3.8 can't be built on Ubuntu 14.04 (Trusty Tahir) and Debian 8 (Jessie) (Azure/azure-cli#20869 (comment)).

In order to move away from Python 3.6, Ubuntu 14.04 (Trusty Tahir) and Debian 8 (Jessie) support should be dropped.

[opbld31]

Docs Build status updates of commit ac7daee:

✅ Validation status: passed

File	Status	Preview URL	Details
docs-ref-conceptual/includes/cli-install-linux-apt.md	✅Succeeded	View

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