Simplfy disable context gc command line arg

This MR aims to simplify the --context-pruning command line option from: --context-pruning Enabled/Disabled option to a --disable-context-pruning flag.

This is not really a breaking change as no Octez version was released with it and as the command was not advertised nor used.

Manually testing the MR

dune exec tezt/tests/main.exe -- -title "Alpha: storage context pruning call" and green CI

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR