[6.x] Pass validation replacements when storing and updating terms

[duncanmcclean]

This pull request fixes an issue where a term blueprint using the new \Statamic\Rules\UniqueTermValue({taxonomy}, {id}, {site}) rule would always fail validation with "This value has already been taken" when saving an existing term.

Statamic already enforces slug uniqueness itself in TermsController, with the real taxonomy, term ID and site passed to the rule. However, you might want to use the rule yourself on a custom field, and the {taxonomy}, {id} and {site} placeholders are the documented way to do that. Those placeholders are swapped in via withReplacements(), which TermsController@update and store never called, so they all resolved to null. With no except value, the rule matched the term being edited and failed. EntriesController and UsersController already pass their own replacements.

This PR fixes it by having TermsController@update and store build their validators via $fields->validator()->withRules([...])->withReplacements([...]), passing id, taxonomy and site on update, and taxonomy and site on store.

Fixes #10078