[498] revert to libsignal v t23_m1

- in m2_WIP, we modified `SignalServiceAddress#identifier` to prefer
  phone numbers to UUIDs (as opposed to preferring UUIDs to phone
  numbers, as the upstream version does)
- we did this in an attempt to attempt to solve a bug that was
  better addressed by making the `phone_number` column on the
  `contacts` table nullable
- here we revert that change it because:
  - (1) it is risky to diverge from mainline signal on this part of
    the data model
  - (2) the change appears to have introduced another bug in which in
    which first-time incoming prekey bundle messages could not create
    a session b/c of a Bad Mac error

signalc/build.gradle.kts

@@ -78,7 +78,7 @@ object Versions {
     const val kotlinSerialization = "1.2.1"
     const val h2 = "1.4.199"
     const val hikariCp = "4.0.3"
-    const val libsignal = "2.15.3_unofficial_t23_m2_WIP"
+    const val libsignal = "2.15.3_unofficial_t23_m1"
     const val liquibase = "4.2.2"
     const val liquibasePlugin = "2.0.4"
     const val logback = "1.2.3"