Update actions/checkout requirement to v2.3.0

Updates the requirements on [actions/checkout](https://github.com/actions/checkout) to permit the latest version.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/master/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/commits/b4483adec309c0d01a5435c5e24eb40de5773ad9)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/build_docs.yml

@@ -14,7 +14,7 @@ jobs:
   build_wiki:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v2.3.0
     - name: Setup Python 3.7
       uses: actions/setup-python@v2.0.1
       with:

.github/workflows/rebase.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: [ubuntu-latest]
     if: github.event.issue.pull_request != '' && contains(github.event.comment.body, '/rebase')
     steps:
-      - uses: actions/checkout@master
+      - uses: actions/checkout@v2.3.0
       - name: Auto Rebase
         uses: cirrus-actions/rebase@1.3.1
         env:

.github/workflows/release_actions.yml

@@ -10,7 +10,7 @@ jobs:
   publish:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v1
+    - uses: actions/checkout@v2.3.0
     - name: Set up JDK 1.8
       uses: actions/setup-java@v1
       with: