Limit `ExponentialBackoff` to specifying just `$backoff` #50

Job	Run time
Validate composer dependencies	10s
Code Coverage	42s
Fix PHP codestyle with php-cs-fixer	23s
Normalize composer.json	14s
PHPStan on PHP 8.1 with Laravel ^9.51	27s
PHPStan on PHP 8.1 with Laravel ^10	24s
Tests - PHP 8.1, Illuminate , lowest	48s
PHPStan on PHP 8.2 with Laravel ^9.51	17s
Tests - PHP 8.1, Illuminate , lowest	36s
Tests - PHP 8.1, Illuminate , highest	35s
Tests - PHP 8.1, Illuminate , highest	34s
PHPStan on PHP 8.2 with Laravel ^10	17s
Tests - PHP 8.2, Illuminate , lowest	46s
PHPStan on PHP 8.3 with Laravel ^9.51	23s
Tests - PHP 8.2, Illuminate , lowest	47s
PHPStan on PHP 8.3 with Laravel ^10	16s
Tests - PHP 8.2, Illuminate , highest	36s
Tests - PHP 8.2, Illuminate , highest	45s
Tests - PHP 8.3, Illuminate , lowest	37s
Tests - PHP 8.3, Illuminate , lowest	42s
Tests - PHP 8.3, Illuminate , highest	34s
Tests - PHP 8.3, Illuminate , highest	44s
	11m 37s

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