Standardize to US English spelling: "canceled" vs "cancelled"

What does this MR do and why?

Standardizes GitLab documentation to consistently use US English spelling for "canceled" and related terms, replacing British spelling variants.

• Word list: Add entry for canceled/cancelled with usage guidance.
• Documentation updates: Replace "cancelled" with "canceled", including the deprecations template (user-facing text).

Additional consideration: Vale rule level

Currently, "cancelled/canceled" is flagged in the SubstitutionWarning rule (warning level). For complete consistency with other British/US spelling pairs, we could consider moving these entries to the British rule (error level).

🔗 References

• Related issue: Replace UI copy instances of "cancelled" with "... (#572555)