Fix tests that fail because validation error message changes in laravel/laravel 8.5.21

[dennis-koster]

When checking out this package and running the tests, some assertions on URL validity error messages fail, if the installed version of orchestra/testbench-core is equal or higher than ^v6.23.1, because of this change:
orchestral/testbench-core@f189915

Or the laravel/laravel equivalent:
laravel/laravel@8665891

To fix this, I have added a more explicit requirement for orchestra/testbench-core in the composer.json file. This is, unfortunately, a breaking change.

[wimski]

@daniel-de-wit do you have an issue with this?

@dennis-koster 8.5.21 is sufficient
https://github.com/laravel/laravel/releases/tag/v8.5.21

[coveralls]

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.611%

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Totals
Change from base Build 1025809167:	0.0%
Covered Lines:	256
Relevant Lines:	257

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