Run tests across .NET versions using a CI matrix

[amarques-intellicure]

Summary

• Runs the .NET Test & Coverage workflow in parallel across .NET 8, 9, and 10 using a GitHub Actions matrix (fail-fast: false).
• Installs one SDK per job via ${{ matrix.dotnet-version }} instead of installing all SDKs in a single job.
• Names test and coverage artifacts per .NET version to avoid collisions between matrix jobs.
• Pins the PostgreSQL test image to postgres:17 and drops unused issues: write workflow permission.

Note: The matrix strategy itself was already merged upstream in #100. This PR includes that behavior plus the small CI hardening tweaks above.

Test plan

• Confirm GitHub Actions runs three parallel jobs for 8.0.x, 9.0.x, and 10.0.x
• Verify all matrix jobs pass on Ubuntu with Testcontainers
• Confirm artifacts are uploaded as test-results-<version>, coverage-report-<version>, and html-coverage-report-<version>
• Confirm coverage summaries appear independently in each job log without conflicts