Pin softprops/action-gh-release to v0.1.15 (#17995)

We are still seeing duplicate releases on v2.0.5, so roll back further.
[Other](https://github.com/Poko-Apps/curl-openssl-android/commit/f8a5a60b7c4b196c703d322bb3d11e9495807426#diff-88ab30345d9874c4336fe50b54b083ba5bdd925be961c34060e6a192b56b0433R72)
[repositories](https://github.com/Glistix/glistix/commit/55fca4fec74aa114faf553b563ae5883b5d76be0#diff-e426ed45842837026e10e66af23d9c7077e89eacbe6958ce7cb991130ad05adaR105)
seem to have settled on this version.

Addresses https://github.com/element-hq/synapse/issues/17991

We're just going to test this during 1.121.0rc1.