Correct Gitaly seccompProfile default in docs

What does this MR do?

The Gitaly parameter table listed RuntimeDefault as the default for securityContext.seccompProfile.type. The chart stopped setting it in 7e5007ae, because the profile blocked the syscalls Gitaly needs to run Git. This MR clears the default and gives the reason in the table row.

Related issues

Relates to #5780 (closed)

Author's checklist

• Ensure the branch name starts with docs-, docs/ or ends with -docs, so only the docs-related CI jobs are included
• Consider taking the GitLab Technical Writing Fundamentals course
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  • Style Guide.
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• ~"feature"
• ~"frontend"
• ~"backend"
• ~"bug"
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These labels cause the MR to be added to code verification QA issues.

Review checklist

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