Remove 0.26.0 deprecations

[kt474]

Summary

Changes in this PR:

• Passing in a backend as a string is no longer valid
• If a backend is passed to sampler/estimator from within an open session context manager, the job will now run from within that session. If the backend passed in is different from the session backend, an error will be raised.
• Service is no longer a valid parameter in Session/Batch
• Refactored Session.from_id() because it used to use service.
• Removed QiskitRuntimeService.global_service because this was only necessary when backends could be given as strings.
• Update tests to support all the above changes

I can also split this PR into multiple if that makes it easier to review

Details and comments

Fixes #1681
Fixes #1727
Fixes #1886