[ci] Added dependabot and upgraded GitHub actions

[pandafy]

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [ ] I have manually tested the changes proposed in this pull request.
• [ ] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [ ] I have updated the documentation.

[github-actions[bot]]

sh-checker report

To get the full details, please check in the job output.

shellcheck errors

``` 'shellcheck ' returned error 1 finding the following syntactical issues: ---------- In openwisp_agent line 693: return $result ^-----^ SC2086 (info): Double quote to prevent globbing and word splitting. Did you mean: return "$result" For more information: https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ... ---------- You can address the above issues in one of three ways: 1. Manually correct the issue in the offending shell script; 2. Disable specific issues by adding the comment: # shellcheck disable=NNNN above the line that contains the issue, where NNNN is the error code; 3. Add '-e NNNN' to the SHELLCHECK_OPTS setting in your .yml action file. ```

shfmt errors

``` 'shfmt ' found no issues. ```