Job Monitor: target NetMinimum and allow runtime roll-forward

[mmitche]

Parent: #17171

Lower the framework-dependent Helix Job Monitor .NET tool from the repository's latest bundled target framework to $(NetMinimum), and allow it to run on newer installed .NET runtimes through major-version roll-forward.

Motivation

The monitor currently targets the latest in-development runtime, which unnecessarily requires that exact runtime on the fixed Azure Pipelines monitor agent. The tool does not require latest-runtime APIs. Targeting Arcade's supported minimum framework while opting into major roll-forward keeps one portable tool package and allows the existing agent SDK/runtime to host it.

Scope

• Target Microsoft.DotNet.Helix.JobMonitor at $(NetMinimum) instead of $(BundledNETCoreAppTargetFramework).
• Set <RollForward>Major</RollForward> for the packaged executable.
• Keep the existing framework-dependent PackAsTool package, manifest installation, and dotnet exec invocation model.
• Align direct project dependencies only where required for the Job Monitor to compile at $(NetMinimum); avoid unrelated framework-target changes.
• Verify the packed tool/runtimeconfig carries the intended target framework and Major roll-forward setting.
• Preserve Job Monitor behavior and existing scenario coverage.

Out of scope

• Self-contained publication.
• RID-specific packages or apphosts.
• Native AOT or single-file publication.
• Pipeline-template installation or invocation changes.

Acceptance criteria

• The Job Monitor builds and packs for $(NetMinimum) as a framework-dependent .NET tool.
• Its generated runtimeconfig targets the minimum framework and specifies major-version roll-forward.
• The installed tool can launch on the monitor agent using a newer available runtime when the minimum runtime is absent.
• Existing Job Monitor tests continue to pass.
• Any dependency target changes are minimal and justified by the executable's dependency graph.

[mmitche]

Draft implementation is available in #17231, stacked on #17223.

[mmitche]

PR #17231 has been rebased directly onto main and is no longer stacked on #17223.