Fix warning for rfc/deprecate-implicitly-nullable-types and Add php version 8.2 ~ 8.4 to strategy matrix

[nukisashineko]

for https://wiki.php.net/rfc/deprecate-implicitly-nullable-types

before:

~/work/Specify$ docker run --rm --interactive --tty --volume $PWD:/app --workdir /app php:8.4-cli vendor/bin/phpunit -c ./phpunit.xml

Deprecated: Codeception\Specify::specify(): Implicitly marking parameter $code as nullable is deprecated, the explicit nullable type must be used instead in /app/src/Codeception/Specify.php on line 20

Deprecated: Codeception\Specify::describe(): Implicitly marking parameter $code as nullable is deprecated, the explicit nullable type must be used instead in /app/src/Codeception/Specify.php on line 30

Deprecated: Codeception\Specify::it(): Implicitly marking parameter $code as nullable is deprecated, the explicit nullable type must be used instead in /app/src/Codeception/Specify.php on line 40

Deprecated: Codeception\Specify::its(): Implicitly marking parameter $code as nullable is deprecated, the explicit nullable type must be used instead in /app/src/Codeception/Specify.php on line 51

Deprecated: Codeception\Specify::should(): Implicitly marking parameter $code as nullable is deprecated, the explicit nullable type must be used instead in /app/src/Codeception/Specify.php on line 56

Deprecated: Codeception\Specify::shouldNot(): Implicitly marking parameter $code as nullable is deprecated, the explicit nullable type must be used instead in /app/src/Codeception/Specify.php on line 67

Deprecated: Codeception\Specify\SpecifyHooks::runSpec(): Implicitly marking parameter $callable as nullable is deprecated, the explicit nullable type must be used instead in /app/src/Codeception/Specify/SpecifyHooks.php on line 38

Deprecated: Codeception\Specify\SpecifyHooks::beforeSpecify(): Implicitly marking parameter $callable as nullable is deprecated, the explicit nullable type must be used instead in /app/src/Codeception/Specify/SpecifyHooks.php on line 207

Deprecated: Codeception\Specify\SpecifyHooks::afterSpecify(): Implicitly marking parameter $callable as nullable is deprecated, the explicit nullable type must be used instead in /app/src/Codeception/Specify/SpecifyHooks.php on line 212
PHPUnit 9.6.22 by Sebastian Bergmann and contributors.

..
Deprecated: Codeception\Specify\SpecifyTest::run(): Implicitly marking parameter $result as nullable is deprecated, the explicit nullable type must be used instead in /app/src/Codeception/Specify/SpecifyTest.php on line 68
I...................

Time: 00:00.038, Memory: 8.00 MB

OK, but incomplete, skipped, or risky tests!
Tests: 22, Assertions: 67, Incomplete: 1.

after

~/work/Specify$ docker run --rm --interactive --tty --volume $PWD:/app --workdir /app php:8.4-cli vendor/bin/phpunit -c ./phpunit.xml
PHPUnit 9.6.22 by Sebastian Bergmann and contributors.

..I...................

Time: 00:00.039, Memory: 8.00 MB

OK, but incomplete, skipped, or risky tests!
Tests: 22, Assertions: 67, Incomplete: 1.