Update example to work with PHP 8

[jeffreyparker]

Description

Updates the example to be compatible with PHP 8 by updating the slim dependencies and fixing a warning with the http_proxy array access. Slim no longer supports PHP 7.3, so also dropping support for that. (PHP 7.3 was EOL 12/2021. PHP 7.4 is also EOL, but still heavily used)

Motivation and Context

The example should work with all currently-supported versions of PHP.

How Has This Been Tested?

Manually verified locally, and added basic example app validation to the CI testing.

Types of Changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

[mbish]

LGTM I just had one question but it's not blocking