Update pandoc docker image to v3.x

When we run the release of chaos-monkey-spring-boot v3.1.1 we got an error during the creation of the GitHub Release, because the pandoc/core image is not available in version 3.13 anymore.

The error was:
```
Pull down action image 'pandoc/core:3.13'
  /usr/bin/docker pull pandoc/core:3.13
  Error response from daemon: manifest for pandoc/core:3.13 not found: manifest unknown: manifest unknown
  Warning: Docker pull failed with exit code 1, back off 4.301 seconds before retry.
```

Author: WtfJoke

.github/workflows/releasetag.yml

@@ -76,7 +76,7 @@ jobs:
       - name: Generate documentation (docbook, html, zip)
         run: ./mvnw asciidoctor:process-asciidoc@output-docbook generate-resources -pl chaos-monkey-docs
       - name: Convert changelog from docbook to markdown
-        uses: docker://pandoc/core:3.13
+        uses: docker://pandoc/core:3
         with:
           args: "-f docbook -t gfm -o changes.md chaos-monkey-docs/target/generated-docbook/changes.xml --wrap=none"
       - name: Generate jar with dependencies