Delete 1.13.0 from CI

For some reasons old rustc 1.18.0 and older started failing to parse the
registry overnight.

    error: An unknown error occurred

    Caused by:
      Feature `pretty` depends on `syntex_syntax` which is not an optional dependency.
    Consider adding `optional = true` to the dependency

.github/workflows/ci.yml

@@ -75,24 +75,6 @@ jobs:
           RUSTFLAGS: --cfg exhaustive ${{env.RUSTFLAGS}}
         if: matrix.os != 'windows'
 
-  msrv:
-    name: Rust 1.13.0
-    runs-on: ubuntu-latest
-    timeout-minutes: 45
-    steps:
-      - uses: actions/checkout@v3
-      - uses: dtolnay/rust-toolchain@1.13.0
-      - name: Get timestamp for cache
-        id: date
-        run: echo yearmo=$(date +%Y%m) >> $GITHUB_OUTPUT
-      - uses: actions/cache@v1
-        with:
-          path: ~/.cargo/registry/index
-          key: cargo-registry-index-${{steps.date.outputs.yearmo}}
-      - run: cd serde && cargo build --features rc
-      - run: cd serde && cargo build --no-default-features
-      - run: cd serde_test && cargo build
-
   build:
     name: Rust ${{matrix.rust}}
     runs-on: ubuntu-latest
@@ -106,8 +88,10 @@ jobs:
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: ${{matrix.rust}}
+      - run: cd serde && cargo build --features rc
       - run: cd serde && cargo build --no-default-features
       - run: cd serde && cargo build
+      - run: cd serde_test && cargo build
 
   more:
     name: Rust ${{matrix.rust}}