Raise msrv to 1.56 (#147)

Necessary as httpdate 1.0.3 introduced MSRV, choosing 1.56

Author: serprex

.github/workflows/ci.yml

@@ -3,7 +3,7 @@ name: CI
 on: [push, pull_request]
 
 env:
-  minrust: 1.51.0
+  minrust: 1.56.0
 
 jobs:
   test:

Cargo.toml

@@ -9,6 +9,7 @@ repository = "https://github.com/hyperium/headers"
 authors = ["Sean McArthur <[email protected]>"]
 keywords = ["http", "headers", "hyper", "hyperium"]
 categories = ["web-programming"]
+rust-version = "1.56"
 
 [workspace]
 members = [