Update GH Action branches.

Author: offa

.github/workflows/ci.yml

@@ -23,15 +23,15 @@ jobs:
       image: "registry.gitlab.com/offa/docker-images/${{ matrix.compiler }}:stable"
     name: "${{ matrix.compiler }}"
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@main
       - name: Build
         run: script/ci_build.sh
 
   build_conan:
     runs-on: ubuntu-20.04
     name: "Conan ${{ matrix.compiler }}"
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@main
       - name: Build
         run: |
           export CONAN_GCC_VERSIONS=10

.github/workflows/codeql.yml

@@ -14,7 +14,7 @@ jobs:
     name: "CodeQL"
     steps:
       - name: Checkout
-        uses: actions/checkout@master
+        uses: actions/checkout@main
       - name: CodeQL Initialization
         uses: github/codeql-action/init@v1
         with: