In GD, change objective_function --> f and deprecate objective_function argument

[MargaretDuff]

Changes

• GD now takes f instead of objective_function to match ISTA and FISTA
• objective_function moved to a kwarg to be deprecated
• Error raised if a user passes both f and objective_function

Testing you performed

Please add any demo scripts to https://github.com/TomographicImaging/CIL-Demos/tree/main/misc

Related issues/links

Closes #2004

Checklist

• I have performed a self-review of my code
• I have added docstrings in line with the guidance in the developer guide
• I have updated the relevant documentation
• I have implemented unit tests that cover any new or modified functionality
• CHANGELOG.md has been updated with any functionality change
• Request review from all relevant developers
• Change pull request label to 'Waiting for review'

Contribution Notes

Please read and adhere to the developer guide and local patterns and conventions.

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in CIL (the Work) under the terms and conditions of the Apache-2.0 License
• I confirm that the contribution does not violate any intellectual property rights of third parties