[internal] upgrade to Rust v1.56.0 (pantsbuild#13321)

Upgrade to Rust v1.56.0.

The [major change](https://blog.rust-lang.org/2021/10/21/Rust-1.56.0.html) is the release of the Rust 2021 edition. This PR does not attempt to switch any Pants crates to 2021 edition though, which can be done piecemeal in separate PRs.

.github/workflows/test-cron.yaml

@@ -52,7 +52,7 @@ jobs:
       uses: actions/cache@v2
       with:
         key: ${{ runner.os }}-rustup-${{ hashFiles('rust-toolchain') }}
-        path: '~/.rustup/toolchains/1.55.0-*
+        path: '~/.rustup/toolchains/1.56.0-*
 
           ~/.rustup/update-hashes
 
@@ -207,7 +207,7 @@ jobs:
       uses: actions/cache@v2
       with:
         key: ${{ runner.os }}-rustup-${{ hashFiles('rust-toolchain') }}
-        path: '~/.rustup/toolchains/1.55.0-*
+        path: '~/.rustup/toolchains/1.56.0-*
 
           ~/.rustup/update-hashes
 

.github/workflows/test.yaml

@@ -52,7 +52,7 @@ jobs:
       uses: actions/cache@v2
       with:
         key: ${{ runner.os }}-rustup-${{ hashFiles('rust-toolchain') }}
-        path: '~/.rustup/toolchains/1.55.0-*
+        path: '~/.rustup/toolchains/1.56.0-*
 
           ~/.rustup/update-hashes
 
@@ -206,7 +206,7 @@ jobs:
       uses: actions/cache@v2
       with:
         key: ${{ runner.os }}-rustup-${{ hashFiles('rust-toolchain') }}
-        path: '~/.rustup/toolchains/1.55.0-*
+        path: '~/.rustup/toolchains/1.56.0-*
 
           ~/.rustup/update-hashes
 
@@ -407,7 +407,7 @@ jobs:
       uses: actions/cache@v2
       with:
         key: ${{ runner.os }}-rustup-${{ hashFiles('rust-toolchain') }}
-        path: '~/.rustup/toolchains/1.55.0-*
+        path: '~/.rustup/toolchains/1.56.0-*
 
           ~/.rustup/update-hashes
 

rust-toolchain

@@ -1,5 +1,5 @@
 [toolchain]
-channel = "1.55.0"
+channel = "1.56.0"
 components = [
   "cargo",
   "clippy",