Backport Specification.unrestricted() from main branch.

[r-gruber]

This would allow 3.5 users to rework their code ahead of the 4.0 release. 'where(null)' was deprecated and should be replaced with unrestricted()

• You have read the Spring Data contribution guidelines.
• You use the code formatters provided here and have them applied to your changes. Don’t submit any formatting related changes.
• You submit test cases (unit or integration tests) that back your changes.
• You added yourself as author in the headers of the classes you touched. Amend the date range in the Apache license header if needed. For new types, add the license header (copy from another file and set the current year only).

[mp911de]

Thank you for your contribution. I've missed that you've already submitted a pull request and so I handled the backport through #3942.