base64ct: MSRV 1.60 (#886)

`proptest` v1.1.0 has an MSRV of 1.60 by way of the `unarray` crate,
which requires `feature(maybe_uninit_extra)`.

We can get rid of all of the MSRV-related build/CI hacks by simply
bumping MSRV to 1.60.

.github/workflows/base64ct.yml

@@ -23,7 +23,7 @@ jobs:
     strategy:
       matrix:
         rust:
-          - 1.56.0 # MSRV
+          - 1.60.0 # MSRV
           - stable
         target:
           - thumbv7em-none-eabi
@@ -37,7 +37,6 @@ jobs:
           target: ${{ matrix.target }}
           override: true
       - uses: RustCrypto/actions/cargo-hack-install@master
-      - run: rm ../Cargo.toml # Hack to preserve MSRV <1.60
       - run: cargo hack build --target ${{ matrix.target }} --feature-powerset --exclude-features std
 
   minimal-versions:
@@ -50,7 +49,7 @@ jobs:
     strategy:
       matrix:
         rust:
-          - 1.56.0 # MSRV
+          - 1.60.0 # MSRV
           - stable
     steps:
       - uses: actions/checkout@v3
@@ -60,5 +59,4 @@ jobs:
           toolchain: ${{ matrix.rust }}
           override: true
       - uses: RustCrypto/actions/cargo-hack-install@master
-      - run: rm ../Cargo.toml # Hack to preserve MSRV <1.60
       - run: cargo hack test --feature-powerset