[TS] [DPG] BearerAuth generates lower case "bearer <key>"

[glharper]

The BearerAuth keyword in TypeSpec seems to generate the following client code in TS:

  if (isKeyCredential(credentials)) {
    client.pipeline.addPolicy({
      name: "customKeyCredentialPolicy",
      async sendRequest(request, next) {
        request.headers.set("Authorization", "bearer " + credentials.key);
        return next(request);
      },
    });
  }

That fifth line should be (note capitalized Bearer): request.headers.set("Authorization", "Bearer " + credentials.key);

[timovv]

Related: #2109