Pin Coverlet.collector to 6.0.4

[jnyrup]

Coverlet.collector 8+ requires .NET 8+
See coverlet-coverage/coverlet#1842

This overrides #119

IMPORTANT

• If the PR touches the public API, the changes have been approved in a separate issue with the "api-approved" label.
• The code complies with the Coding Guidelines for C#.
• The changes are covered by unit tests which follow the Arrange-Act-Assert syntax and the naming conventions such as is used in these tests.
• If the PR changes the public API the changes needs to be included by running AcceptApiChanges.ps1 or AcceptApiChanges.sh.