feat(api): add Delete and Sub methods, update CI workflows

- Add Delete(key) to remove keys from all config layers
- Add Sub(key) to get a scoped ConfigManager for nested sections
- Update CodeQL workflow actions to v4/v5/v3
- Update govulncheck workflow checkout to v4
- Remove boilerplate comments from CodeQL workflow
- Add comprehensive tests for both new methods
- Update README with Sub() usage example and API docs

.github/workflows/codeql-analysis.yml

@@ -36,31 +36,20 @@ jobs:
 
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
 
     - name: Install Go
-      uses: actions/setup-go@v4
+      uses: actions/setup-go@v5
       with:
         go-version-file: go.mod
 
     - name: Initialize CodeQL
-      uses: github/codeql-action/init@v2
+      uses: github/codeql-action/init@v3
       with:
         languages: ${{ matrix.language }}
 
     - name: Autobuild
-      uses: github/codeql-action/autobuild@v2
-
-    # ℹ️ Command-line programs to run using the OS shell.
-    # 📚 https://git.io/JvXDl
-
-    # ✏️ If the Autobuild fails above, remove it and uncomment the following three lines
-    #    and modify them (or add more) to build your code if your project
-    #    uses a compiled language
-
-    #- run: |
-    #   make bootstrap
-    #   make release
+      uses: github/codeql-action/autobuild@v3
 
     - name: Perform CodeQL Analysis
-      uses: github/codeql-action/analyze@v2
+      uses: github/codeql-action/analyze@v3

.github/workflows/govulncheck.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     name: Run govulncheck
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - id: govulncheck
         uses: golang/govulncheck-action@v1
         with: