SpinBox is missing context_menu_enabled property

### Tested versions

v4.2.2.stable.official [15073afe3]

### System information

Ubuntu

### Issue description

`LineEdit` comes with a `context_menu_enabled` property that allows the user to disable the right-click context menu.

`SpinBox` also does have a right-click context menu but appears to be missing the corresponding option to disable it.

I am currently working on a project where right-click context menu must be disabled as they'd cause issues.

### Steps to reproduce

See issue description.

### Minimal reproduction project (MRP)

n/a

Start by comparing the existing context-menu handling in SpinBox with LineEdit's context_menu_enabled property, then locate the relevant UI control implementations. Done means SpinBox exposes the corresponding option and disabling it prevents the right-click context menu on Ubuntu; add or update regression coverage if the surrounding implementation provides tests.

Written by the indexing model from the issue text.