chore: drop 7.3, add 8.2

[Justintime50]

Description

Drops support for PHP 7.3 and adds CI checks for PHP 8.2

Testing

Pull Request Type

Please select the option(s) that are relevant to this PR.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Improvement (fixing a typo, updating readme, renaming a variable name, etc)

[Justintime50]

For whatever reason, CI is failing only on PHP 8.2 with the following which is NOT helpful at all:

Run EASYPOST_TEST_API_KEY=123 EASYPOST_PROD_API_KEY=123 make coverage
composer coverage
> XDEBUG_MODE=coverage ./bin/phpunit --coverage-html clover.html --coverage-clover build/logs/clover.xml
PHPUnit 9.5.26 by Sebastian Bergmann and contributors.

Runtime:       PHP [8](https://github.com/EasyPost/easypost-php/actions/runs/3652991093/jobs/6171968921#step:7:9).2.0 with PCOV 1.0.11
Configuration: /home/runner/work/easypost-php/easypost-php/phpunit.xml.dist

Segmentation fault (core dumped)
Script XDEBUG_MODE=coverage ./bin/phpunit --coverage-html clover.html --coverage-clover build/logs/clover.xml handling the coverage event returned with error code 13[9](https://github.com/EasyPost/easypost-php/actions/runs/3652991093/jobs/6171968921#step:7:10)
make: *** [Makefile:[11](https://github.com/EasyPost/easypost-php/actions/runs/3652991093/jobs/6171968921#step:7:12): coverage] Error 139
.
Error: Process completed with exit code 2.

PHP 8.2 installs correctly; however, when attempting to generate coverage it fails -- Tried swapping the coverage tool from xdebug to pcov and back without luck. I also downgraded Ubuntu thinking that could be it with no luck. I switched the target from coverage to test thinking it was the coverage reporting that was broken but it wouldn't load normal tests either so it's not a coverage generation issue.

[Justintime50]

I'd like to move forward with this PR as is. I was able to test PHP 8.2 locally and it works as expected, just not via our unit tests. I've created a follow-up task to look into that so as to not block this PRs progress.