fix: wrong content type in request

[grafvonb]

  private HttpPost buildRequest() throws URISyntaxException {
    HttpPost httpPost = new HttpPost(jwtCredential.authUrl().toURI());
    httpPost.addHeader("Content-Type", "application/json");

application/json causes java.lang.NullPointerException: Cannot invoke "java.lang.Integer.intValue()" because the return value of "io.camunda.operate.auth.TokenResponse.getExpiresIn()" is null

should be application/x-www-form-urlencoded

[grafvonb]

/assign @jonathanlukas

[jonathanlukas]

@grafvonb thank you for this fix. I will go ahead and merge it.

Please consider creating fixes on top of main as we can backport them to the release branches from there. I will port-forward this fix for now.