rgw: Remove invalid Content-Type header RGW OIDC discovery requests

rgw: Remove invalid Content-Type header RGW OIDC discovery requests

RGW sends OIDC discovery and JWKS requests with an incorrect application/x-www-form-urlencoded Content-Type header, causing 415 (Unsupported Media Type) responses from some identity providers including AWS EKS. This PR removes this header for the GET requests as it was a hold-over from a previous change where the endpoint was previously an HTTP POST https://github.com/ceph/ceph/commit/7566664f89be062e0c9f3519dc60b94c8af5e2a4 and was carried through to the current implementation https://github.com/ceph/ceph/commit/803570cd4f7ffbb3d7a329b0b9450748b3ca335d

Fixes: https://tracker.ceph.com/issues/73877
Signed-off-by: Jonathan Suever <suever@gmail.com>