Carta-casacore upgrade checklist

carta-casacore upgrades should be synchronised with CARTA releases, so that existing backend packages don't have to be rebuilt.

1. Create a carta-casacore PR and a corresponding carta-backend PR.
2. Update the casa6 submodule and its nested casa6/casatools/casacore submodule.
3. Carefully compare the root CMakeLists.txt file to casa6/casatools/casacore/CMakeLists.txt and merge in any required changes (at least the project version and the SOVERSION). We have removed some irrelevant conditional sections, changed some default variable values, modified some paths because of the relative location of the file, and added elements such as the custom imageanalysis build. Apart from that, the files should be kept in sync as far as possible.
4. Check the casa6 submodule's git tags to get the CASA version. Update the SOVERSION comment in imageanalysis/CMakeLists.txt and bump the CASA_PROJECT_SOVERSION if the minor version has changed. Other changes to this file should typically not be required, as sources and headers are found dynamically with a glob.
5. Test building carta-casacore, and make any required changes.
6. Test building the backend against the new carta-casacore, and make any required changes.
7. Merge the carta-casacore PR.
8. Build the carta-casacore packages and upload them to the testing repositories.
9. Update CI to use these packages.
10. Confirm that the backend PR passes tests with the updated CI.
11. Merge the backend PR.
12. Merge dev into all open PRs, and proceed with the release.
13. When the release is complete, make sure that CI is configured to use the preview repositories.