CI: add v1.19 branch to Dependabot config (#5294)

**Which issue(s) this PR fixes**: 
Fixes #

**What this PR does / why we need it**: 
This PR adds the `v1.19` branch to the Dependabot configuration for
GitHub Actions.
To ensure that the maintenance branch (`v1.19`) is also kept up-to-date
with the latest action updates, similar to the `master` branch.

Ref.
https://docs.github.com/en/code-security/tutorials/secure-your-dependencies/customizing-dependabot-prs#targeting-pull-requests-against-a-non-default-branch

**Docs Changes**:
N/A

**Release Note**: 
N/A

---------

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>
Co-authored-by: Kentaro Hayashi <kenhys@gmail.com>

Author: Watson1978

.github/dependabot.yml

@@ -1,6 +1,14 @@
 version: 2
 updates:
+  # master branch
   - package-ecosystem: 'github-actions'
     directory: '/'
     schedule:
-      interval: 'weekly'
+      interval: 'monthly'
+
+  # v1.19 branch
+  - package-ecosystem: 'github-actions'
+    directory: '/'
+    target-branch: 'v1.19'
+    schedule:
+      interval: 'monthly'