Updated test dependencies to fix generated mock class type annotation error

[danielmorell]

Description of the change

This update increases our minimum required dependencies for two of our test dependencies: phpunit/phpunit and mockery/mockery. Without this we see the following error when trying to install and test the lowest versions of all of our dependencies. See #624 for the failing test results.

PHP Fatal error:  Declaration of Mockery\Adapter\Phpunit\MockeryPHPUnitIntegration::assertPostConditions() 
must be compatible with PHPUnit\Framework\TestCase::assertPostConditions(): void in 
/home/runner/work/rollbar-php/rollbar-php/tests/ConfigTest.php on line 37

Updating only mockery/mockery solves the above issue. However, it introduces a new one. The mock generated from the Data class in...

https://github.com/rollbar/rollbar-php/blob/e7bb5f91921baf4a0e9d08e510c7791aa22b0d5a/tests/VerbosityTest.php#LL397C1-L399C1

Results in one of the generated methods having a type annotation both a ? and |null which is invalid and causes a fatal error during the test run. By updating our version of phpunit/phpunit that issue is resolved.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Maintenance
• New release

Related issues

None

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers assigned
• Issue from task tracker has a link to this pull request
• Changes have been reviewed by at least one other engineer

[waltjones]

👍