ci: bump MSRV to Rust 1.60

winapi-util has bumped its MSRV, but termcolor still works fine with
winapi-util <0.1.6.

(I didn't bother figuring out winapi-util's actual MSRV, but it does at
least build with Rust 1.60. So it's unlikely to be a problem in
practice.)

<... after a few rounds of CI failures ...>

OK, for some reason, I can't successfully pin winapi-util to 0.1.5
locally or in CI. It is senseless to burn more time on it, so let's just
set the pinned version to Rust 1.60 and be done with it.

.github/workflows/ci.yml

@@ -12,22 +12,13 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        build:
-        - pinned
-        - pinned-win
-        - stable
-        - beta
-        - nightly
-        - macos
-        - win-msvc
-        - win-gnu
         include:
         - build: pinned
           os: ubuntu-latest
-          rust: 1.34.0
+          rust: 1.60.0
         - build: pinned-win
           os: windows-latest
-          rust: 1.34.0
+          rust: 1.60.0
         - build: stable
           os: ubuntu-latest
           rust: stable