srtp: 1.5.4 -> 2.1.0

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 2.1.0 with grep in /nix/store/rgqp3xyxn4m88whigf4msz3kj07hhgkf-libsrtp-2.1.0
- directory tree listing: https://gist.github.com/d1a145c8a1c6845fe2715b901b9f99b2