[ci] Added dependabot and upgraded GitHub actions

[pandafy]

Checklist

• 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]

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.