Deprecate use-rust.yml step template

[heaths]

Now that #4486 is merged, we should look at either removing or limiting use of eng/pipelines/templates/steps/use-rust.yml. It seems to be wrong in some uses, deciding on 1.92 but then we end up using a stable or nightly pinned toolchain making that previous rustup set active call useless and even deceptive when diagnosing pipeline failures.

I had copilot build a call graph and where the Toolchain parameter is set:

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Top-level pipeline roots under eng/pipelines that reach use-rust.yml: pr.yml and pullrequest.yml. prepare-pipelines.yml does not. There is also one standalone template path via templates/jobs/perf.yml, but nothing under eng/pipelines references it.

• eng/pipelines/pullrequest.yml
• templates/stages/archetype-sdk-client.yml - templates/jobs/ci.yml - templates/jobs/pack.yml - templates/steps/use-rust.yml — Toolchain not passed → defaults to active
• templates/steps/use-rust.yml — Toolchain: $(PackToolchain) → 1.92
• templates/jobs/analyze.yml — ci.yml sets Toolchain*#COLON|* stable - templates/steps/use-rust.yml — Toolchain: nightly (conditional: only when analyze toolchain != nightly)
• templates/steps/use-rust.yml — Toolchain: stable
• eng/common/pipelines/templates/jobs/generate-job-matrix.yml - templates/jobs/ci.tests.yml - templates/steps/use-rust.yml — Toolchain*#COLON|* $(RustToolchainName) - from templates/stages/pr-platform-matrix.json
• values used: stable on Ubuntu/Windows/macOS; msrv and nightly only on Ubuntu
• eng/common/pipelines/templates/jobs/generate-job-matrix.yml - templates/jobs/live.tests.yml - templates/steps/use-rust.yml — Toolchain: $(RustToolchainName)
• this path exists in the hierarchy, but pullrequest.yml sets RunLiveTests: false
• templates/stages/archetype-rust-release.yml - templates/steps/use-rust.yml — Toolchain: nightly
• this path is present in the template, but pullrequest.yml sets IncludeRelease: false
• eng/pipelines/pr.yml
• same template chain as pullrequest.yml
• differences: - MatrixConfigs comes from archetype-sdk-client.yml defaults, so ci.tests.yml gets $(RustToolchainName) = stable | msrv | nightly from templates/stages/platform-matrix.json
• RunLiveTests is not set here, so it stays false by default
• IncludeRelease stays true by default, so the release path is not disabled by parameter, but it is still gated by the stage conditions in archetype-sdk-client.yml
• Standalone template path not referenced by another file in eng/pipelines
• templates/jobs/perf.yml - eng/common/pipelines/templates/jobs/perf.yml - InstallLanguageSteps - templates/steps/use-rust.yml — Toolchain: stable

Templates that set Toolchain on the way to use-rust.yml:

• templates/jobs/ci.yml → stable for analyze.yml
• templates/jobs/analyze.yml → nightly, then ${{ parameters.Toolchain }}
• templates/jobs/pack.yml → omitted once, then $(PackToolchain) = 1.92
• templates/jobs/ci.tests.yml → $(RustToolchainName)
• templates/jobs/live.tests.yml → $(RustToolchainName)
• templates/stages/archetype-rust-release.yml → nightly
• templates/jobs/perf.yml → stable