PMM-7 Output trivy reports in table/text format

[ademidoff]

This pull request updates the Trivy image scanning step in the pmm/v3/pmm3-image-scanning.groovy pipeline to change the output format of the main vulnerability report from JSON to table format.

Image scanning report format change:

• The Trivy command now outputs the main vulnerability report in table format to trivy-report.txt instead of JSON format to trivy-report.json, which allows to see the sources of CVEs.