bump toolchain for cargo-msrv (#1046)

Author: tomtau

.github/workflows/ci.yml

@@ -79,7 +79,7 @@ jobs:
         with:
           kind: msrv
           tools: cargo-msrv
-          toolchain: 1.73.0 # needed for cargo-msrv due to cargo-platform v0.1.8
+          toolchain: 1.78.0 # needed for cargo-msrv
       - name: Check msrv
         shell: sh
         run: for crate in "derive" "generator" "grammars" "meta" "pest" "vm"; do cd "$crate" && cargo msrv verify && cd ..; done