Allow disabling of "auto create label if none exists" behaviour

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From this comment #415036 (comment 1649485189) in #415036 (closed):

I feel like our API should not be auto-creating labels if they don't exist.

I don't think this behavior is even documented. But this behavior has been in the codebase for so long that users probably depend on this already. 🤔

• Introduce an argument force_new_label_creation which defaults to true (which preserves the current behaviour) but can be set to false
• Add a breaking change entry to https://docs.gitlab.com/ee/api/rest/deprecations.html so we can ensure we change the default behaviour for REST v5.