Clarify Dependabot cooldown wording in tutorial

[atharvasingh7007]

Summary

• align the tutorial wording for cooldown with the Dependabot options reference
• clarify that cooldown defines a cooldown period for dependency updates
• state explicitly that cooldown applies to version updates, not security updates

Closes #43708.

Validation

• npm run lint-content -- --paths content/code-security/tutorials/secure-your-dependencies/optimizing-pr-creation-version-updates.md

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Source	Review	Production	What Changed
code-security/tutorials/secure-your-dependencies/optimizing-pr-creation-version-updates.md	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14	fpt ghec ghes@ 3.20 3.19 3.18 3.17 3.16 3.15 3.14

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