Bump strata-primitives from 42b8e1d to 3339f7f in /backend

Bumps [strata-primitives](https://github.com/alpenlabs/alpen) from `42b8e1d` to `3339f7f`.
- [Release notes](https://github.com/alpenlabs/alpen/releases)
- [Commits](alpenlabs/alpen@42b8e1d...3339f7f)

---
updated-dependencies:
- dependency-name: strata-primitives
  dependency-version: 3339f7fc7ef2dd48456e4693a05dbc3c131e3503
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

backend/Cargo.lock

@@ -26,7 +26,7 @@
   strata-bridge-rpc = { git = "https://github.com/alpenlabs/strata-bridge.git", features = [
     "client",
   ], tag = "v0.2.0-rc3" }
-  strata-primitives = { git = "https://github.com/alpenlabs/alpen.git", tag = "v0.2.0-rc4" }
+  strata-primitives = { git = "https://github.com/alpenlabs/alpen.git", tag = "v0.2.0-rc6" }
 
 [dev-dependencies]
   mockito = "1.6.1"