Google suggests that libsrtp is maintained in Cisco's Github repo here:
And, if you browse around there, you can find links to the downloadable packages:
If I scroll down that page, I find a link to the .tar.gz package:
Building solutions based on toolkits is going to require you to be resourceful. When people make suggestions to you like "The latest supported version is 1.5.4, I would suggest updating to that," the best thing you can do is some self-searching to see if you can find it. And, if you're unsure about what you find, but asking as a question with what you've been able to learn, e.g.
"Is this (https://github.com/cisco/libsrtp/archive/v1.5.4.tar.gz) the correct thing to download?"
people are pleased to continue trying to help.