Update actions/checkout to v7.

Author: orium

.github/workflows/ci.yml

@@ -33,7 +33,7 @@ jobs:
           cargo install cargo-deadlinks
 
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v7
 
       - name: Check everything
         run: bash ./tools/check.sh basic doc_url_links unused_deps packaging fmt toml_fmt readme
@@ -56,7 +56,7 @@ jobs:
         run: cargo install cargo-msrv
 
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v7
 
       - name: Check the minimum supported rust version
         run: bash ./tools/check.sh msrv
@@ -71,7 +71,7 @@ jobs:
           components: clippy
 
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v7
 
       - name: Run clippy
         run: bash ./tools/check.sh clippy