[release] Update version to 3.4-SNAPSHOT

Note: version 3.4-SNAPSHOT was already pushed earlier via 46cbfd546ef2600efb3ceda77827d3989d952767, but version 3.3 was never released when this commit was pushed.
diff --git a/docs/config.toml b/docs/config.toml
index 3fbac48..4a28508 100644
--- a/docs/config.toml
+++ b/docs/config.toml
@@ -61,6 +61,7 @@
   ]
 
   PreviousDocs = [
+    ["3.3", "https://nightlies.apache.org/flink/flink-statefun-docs-release-3.3"],
     ["3.2", "https://nightlies.apache.org/flink/flink-statefun-docs-release-3.2"],
     ["3.1", "https://nightlies.apache.org/flink/flink-statefun-docs-release-3.1"],
     ["3.0", "https://nightlies.apache.org/flink/flink-statefun-docs-release-3.0"],