chore(lib): bump MSRV to 1.56 (#2902)

(cherry picked from commit a563404)

Author: seanmonstar

.github/workflows/CI.yml

@@ -101,7 +101,7 @@ jobs:
     strategy:
       matrix:
         rust:
-          - 1.49 # keep in sync with MSRV.md dev doc
+          - 1.56 # keep in sync with MSRV.md dev doc
 
         os:
           - ubuntu-latest

docs/MSRV.md

@@ -6,4 +6,4 @@ hyper. It is possible that an older compiler can work, but that is not
 guaranteed. We try to increase the MSRV responsibly, only when a significant
 new feature is needed.
 
-The current MSRV is: **1.49**.
+The current MSRV is: **1.56**.