build-aux: Revert libsrt to v1.5.3

With libsrt v1.5.4, we saw issues when using multiple Media Sources
ingesting SRT. If one would shut down or fail, then all SRT Media
Sources would stop working. While there has been no confirmation that
this also occurs on Linux, out of an abundance of caution, revert libsrt
to v1.5.3, which was used for OBS Studio 30.1, 30.2, and 31.0.

build-aux/modules/20-srt.json

@@ -16,8 +16,8 @@
         {
             "type": "git",
             "url": "https://github.com/Haivision/srt.git",
-            "tag": "v1.5.4",
-            "commit": "a8c6b65520f814c5bd8f801be48c33ceece7c4a6"
+            "tag": "v1.5.3",
+            "commit": "09f35c0f1743e23f514cb41444504a7faeacf89e"
         }
     ]
 }