Fix typos in GUIDELINES.md and SECURITY.md

[Dimitrolito]

This pull request addresses minor typographical errors in the GUIDELINES.md and SECURITY.md files:

• In GUIDELINES.md, corrected the spelling of "mission-critical."
• In SECURITY.md, replaced "on-going" with the correct form "ongoing."

These changes improve clarity and adhere to proper English usage.

PR Checklist:

• Tests
• Documentation
• Changeset entry

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Latest commit: 7aa7539

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