Update CI action versions and Swift matrix

[leogdion]

Problem

The CI workflow (SundialKitStream.yml) is using outdated action versions. Bring it in line with the current versions used in AtLeast and other BrightDigit repos.

Required Updates

Action version bumps

Action	Current	Target
actions/checkout	v4	v6
brightdigit/swift-build	v1.4.0	v1.5.2
sersoft-gmbh/swift-coverage-action	v4	v5
codecov/codecov-action	v4	v6
jdx/mise-action	v2 (pinned 2024.11.0)	v4 (no pin)

Swift version matrix

Add Swift 6.3 to the Ubuntu build matrix. Remove 6.1 and 6.2 nightly variants if no longer needed.

Coverage guard

Wrap coverage steps with if: steps.build.outputs.contains-code-coverage == 'true' (output from swift-build@v1.5.2) to avoid failing on targets with no tests.

Reference

See .github/workflows/atleast.yml in brightdigit/AtLeast for the current pattern.