Build using .Net 9 SDK rather than 8

[jas88]

Proposed Change

• Update SDK used
• Make target .Net version centrally configured from Directory.Build.props instead of per-project
• Enable Dependabot auto-update PRs for the global.json file governing which SDK is used on Github
• Fix WFO1000 errors which are now flagged as serious by .Net - details: [Breaking change]: New security analyzers in WinForms prevent leaking of user data dotnet/docs#42724
• Disambiguate reference to License object (used internally in RDMP, but .Net's System.ComponentModel now has a License object too)

Type of change

What types of changes does your code introduce? Tick all that apply.

• Bugfix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that would cause existing functionality to not work as expected)
• Documentation-Only Update
• Other (if none of the other choices apply)

Checklist

By opening this PR, I confirm that I have:

• Ensured that the PR branch is in sync with the target branch (i.e. it is automatically merge-able)
• Created or updated any tests if relevant
• Have validated this change against the Test Plan
• Requested a review by one of the repository maintainers
• Have written new documentation or updated existing documentation to detail any new or updated functionality and how to use it
• Have added an entry into the changelog