Bump dashmap version in Cargo.toml (#650)

We're using APIs that are only available in v5.5 (e.g. .insert() is
used in core/src/worker/activities.rs). In practice, when compiling
sdk-core alone, v5.5 was already what was being picked up and there
was no problem. However, when sdk-core is compiled as a crate in the
sdk-typescript core-bridge project, v5.4 is getting picked up, with
the result that sdk-core fails to build in that context.

Author: dandavison

core/Cargo.toml

@@ -28,7 +28,7 @@ async-trait = "0.1"
 base64 = "0.21"
 console-subscriber = { version = "0.1", optional = true }
 crossbeam = "0.8"
-dashmap = "5.0"
+dashmap = "5.5"
 derive_builder = "0.12"
 derive_more = "0.99"
 enum_dispatch = "0.3"