Update gencolormap to version 2.4

[marlam]

Please confirm your submission meets all the criteria

• I have read the App Requirements and App Maintenance pages.
• My pull request follows the instructions at App Submission.
• I am using only the minimal set of permissions. (If not, please explain each non-standard permission.)
• All assets referenced in the manifest are redistributable by any party. If not, the unredistributable parts are using an extra-data source type.
• I am an upstream contributor to the project. If not, I contacted upstream developers about submitting their software to Flathub. Link:
• I own the domain used in the application ID or the domain has a policy for delegating subdomains (e.g. GitHub, SourceForge).
• Any additional patches or files have been submitted to the upstream projects concerned. (If not, explain why.)

[flathubbot]

Started test build 125704

[flathubbot]

Build 125704 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/108664/de.marlam.gencolormap.flatpakref