Cannot use 307/308 for HTTPRequestRedirectPolicy StatusCode

**What steps did you take and what happened:**
When using HTTPRequestRedirectPolicy in HTTPProxy you can only set the StatusCode to 301 or 302 but cannot use 307 or 308 for preserving the HTTP method.

**What did you expect to happen:**
307 and 308 values are allowed.

**Environment:**

- Contour version: 1.30
