Update dependency orchestra/testbench to v6.21.1

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
orchestra/testbench (source)	require-dev	minor	^6.20 -> 6.21.1

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Release Notes

orchestral/testbench

v6.21.1

Compare Source

Changes

• Update minimum support for Testbench Core v6.25.2+. (v6.25.0...v6.25.2)

Testbench Changes

Changes

• Ability to use App\Http\Kernel and App\Console\Kernel via Commander if the class exists.

v6.21.0

Compare Source

Changes

• Update minimum support for Testbench Core v6.25.0+. (v6.24.1...v6.25.0)

Testbench Changes

Added

• Add ability to define database seeder using TestCase::defineDatabaseSeeders() method.

Changes

• Update skeleton to match v8.6.2.

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