chore: add monthly Dependabot and harden checkout (#2713)

## What this changes
This PR adds Dependabot configuration and hardens GitHub Actions
checkout behavior.

## Dependabot
- adds .github/dependabot.yml
- enables grouped dependency updates
- schedules updates monthly
- configures a 7-day cooldown to avoid rapid follow-up bumps
- includes GitHub Actions updates

Configured update scopes:
- pip in /
- github-actions in /

## GitHub Actions hardening
Sets persist-credentials: false for ctions/checkout@v4 in:
- .github/workflows/windows_exe.yml
- .github/workflows/packages.yml
- .github/workflows/lint.yml
- .github/workflows/documentation.yml

## Why
- grouped updates reduce PR noise
- monthly cadence is lower maintenance
- cooldown helps avoid update churn
- disabling persisted checkout credentials follows least-privilege CI
practices

---------

Co-authored-by: Alexander Graf <17130992+aandergr@users.noreply.github.com>

.github/workflows/lint.yml

@@ -12,9 +12,11 @@ jobs:
         python-version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
     steps:
       - name: Checkout Instaloader Repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v7
+        with:
+          persist-credentials: false
       - name: Setup Python
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@v6
         with:
           python-version: ${{ matrix.python-version }}
           cache: 'pipenv'