docs: Secure Boot V2: explain the removal of unused keys (IDFGH-13685)

[Lupindakaas]

This PR updates some of the Secure Boot V2 documentation to explain the removal of unused keys.

Related

https://docs.espressif.com/projects/esp-idf/en/v5.2.2/esp32s3/security/secure-boot-v2.html#multiple-keys

Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
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• Git history is clean — commits are squashed to the minimum necessary.

[github-actions]

Warnings

⚠️

Some issues found for the commit messages in this PR: the commit message "Doc: Update secure-boot-v2.rst": type/action should start with a lowercase letter type/action should be one of [change, ci, docs, feat, fix, refactor, remove, revert, test] Please fix these commit messages - here are some basic tips: follow Conventional Commits style correct format of commit message should be: <type/action>(<scope/component>): <summary>, for example fix(esp32): Fixed startup timeout issue allowed types are: change,ci,docs,feat,fix,refactor,remove,revert,test sufficiently descriptive message summary should be between 20 to 72 characters and start with upper case letter avoid Jira references in commit messages (unavailable/irrelevant for our customers) TIP: Install pre-commit hooks and run this check when committing (uses the Conventional Precommit Linter).

	Messages
📖	You might consider squashing your 3 commits (simplifying branch history).

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