ci: pin hashbrown for msrv job (#814)

Author: seanmonstar

.github/workflows/CI.yml

@@ -92,6 +92,8 @@ jobs:
         run: |
           cargo update --package tokio --precise 1.38.1
           cargo update --package tokio-util --precise 0.7.11
+          cargo update --package hashbrown --precise 0.15.0
+
 
       - run: cargo check -p h2