fix: use uppercase format for x-frame-options header value

[dlouzan]

Description

According to the RFCs, the value of header X-Frame-Options can only be SAMEORIGIN or DENY, both uppercase. Although header names are case-insensitive, header values are not.

References:

• https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options
• https://html.spec.whatwg.org/multipage/document-lifecycle.html#the-x-frame-options-header

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.

[char0n]

Thanks for the PR! You're absolutely right.