[server] Small clean-up in WMS renderer

[pblottiere]

Description

This PR makes some clean-up in WMS renderer and especially in the createImage and configureMapSettings methods.

To compare WMS versions, the QgsProjectVersion class is used and the operator >= has been fixed to be properly working.

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include fixes #11111 in the commit message next to the description
• Commits which add new features are tagged with [FEATURE] in the commit message
• Commits which change the UI or existing user workflows are tagged with [needs-docs] in the commit message and containt sufficient information in the commit message to be documented
• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit

[rldhont]

thanks @pblottiere