Remove deprecated methods in JsonWebSignature

[elharo]

These have been deprecated for 5 years. We promise 18 months before removal. Time for them to go.

     * <p>@deprecated Since release 1.19.1, replaced by {@link #getX509Certificates()}.
     */
    @Deprecated
    public final String getX509Certificate() {
      if (x509Certificates == null || x509Certificates.isEmpty()) {
        return null;
      }
      return x509Certificates.get(0);
    }