CI: update actions, add dependabot (dart-lang/markdown#469)

pkgs/markdown/.github/dependabot.yaml

diff --git a/pkgs/markdown/.github/dependabot.yaml b/pkgs/markdown/.github/dependabot.yaml
new file mode 100644
index 0000000..2144819
--- /dev/null
+++ b/pkgs/markdown/.github/dependabot.yaml
@@ -0,0 +1,8 @@
+# Dependabot configuration file.
+version: 2
+
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "monthly"

pkgs/markdown/.github/workflows/test-package.yml

diff --git a/pkgs/markdown/.github/workflows/test-package.yml b/pkgs/markdown/.github/workflows/test-package.yml
index 1712bfe..4e90396 100644
--- a/pkgs/markdown/.github/workflows/test-package.yml
+++ b/pkgs/markdown/.github/workflows/test-package.yml
@@ -22,8 +22,8 @@
       matrix:
         sdk: [dev]
     steps:
-      - uses: actions/checkout@v2
-      - uses: dart-lang/setup-dart@v1.0
+      - uses: actions/checkout@93ea575cb5d8a053eaa0ac8fa3b40d7e05a33cc8
+      - uses: dart-lang/setup-dart@6a218f2413a3e78e9087f638a238f6b40893203d
         with:
           sdk: ${{ matrix.sdk }}
       - id: install
@@ -49,8 +49,8 @@
         os: [ubuntu-latest]
         sdk: [2.17.0, dev]
     steps:
-      - uses: actions/checkout@v2
-      - uses: dart-lang/setup-dart@v1.0
+      - uses: actions/checkout@93ea575cb5d8a053eaa0ac8fa3b40d7e05a33cc8
+      - uses: dart-lang/setup-dart@6a218f2413a3e78e9087f638a238f6b40893203d
         with:
           sdk: ${{ matrix.sdk }}
       - id: install