switch to standard X-Forwarded-Proto instead of X-Forwarded-Protocol

[xini]

Description

SS6: Silverstripe currently uses the non-standard X-Forwarded-Protocol header. This PR changes that to X-Forwarded-Proto.

See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-Proto.

See SS4 pull request (closed): #10018

Additional context or points of discussion

No response

Validations

• You intend to implement the feature yourself
• You have read the contributing guide
• You strongly believe this feature should be in core, rather than being its own community module
• You have checked for existing issues or pull requests related to this feature (and didn't find any)