Remove Task's `type: 'custom'`

[sheremet-va]

Clear and concise description of the problem

The custom type is identical to the test type; the only difference is that it's initiated by calling getCurrentSuite().task(). Users of the API can attach their own custom properties to differentiate between built-in tests and custom tests.

Suggested solution

• Deprecate the Custom type now in 2.2
• Always set type to 'test' in 2.3
• Remove the Custom in Vitest 3

Alternative

No response

Additional context

No response

Validations

• [X] Follow our Code of Conduct
• [X] Read the Contributing Guidelines.
• [X] Read the docs.
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[sheremet-va]

We might also be able to remove the type = 'custom' before Vitest 3 🤔 (By replacing it with type = 'test') I am not sure how breaking it is 🤔