[PCUI] Add upgrade-insecure-requests directive to fix CSP header

[hehe7318]

Description

Add the upgrade-insecure-requests directive required by AWS Console security standards.

This directive instructs browsers to automatically rewrite any insecure HTTP resource URLs to HTTPS before making the request, preventing mixed content issues and ensuring all resource loads go over encrypted connections.

How Has This Been Tested?

• Backend tests passed
• Manually deployed PCUI and tested function, worked well.

References

PR Quality Checklist

• I added tests to new or existing code
• I removed hardcoded strings and used react-i18next library (useTranslation hook and/or Trans component), see an example here
• I made sure no sensitive info gets logged at any time in the codebase (see here) (e.g. no user info or details, no stacktraces, etc.)
• I made sure that any GitHub issue solved by this PR is correctly linked
• I checked that infrastructure/update_infrastructure.sh runs without any error
• I checked that npm run build builds without any error
• I checked that clusters are listed correctly
• I checked that a new cluster can be created (config is produced and dry run passes)
• I checked that login and logout work as expected

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