chore: pin Elsa / Elsa Studio 3.9.0 stable on release/3.9.0

[sfmskywalker]

Bump-only for the 3.9.0 ext cut, requested by RM. QA'd base tip: b597739.

• ElsaVersion 3.9.0-preview.5757 → 3.9.0
• ElsaStudioVersion 3.9.0-preview.1797 → 3.9.0

Those are the only Elsa / Elsa Studio pins: every Elsa.* and Elsa.Studio.* entry in Directory.Packages.props uses these two properties. I checked all 47 referenced packages, and each returns 200 on nuget.org at 3.9.0.

[sfmskywalker]

Elsa 3 Code Review: APPROVE + HIGH @ 75308d9

Code Review, Round 1/4

Scope: the version change in Directory.Build.props on release/3.9.0: ElsaVersion goes from 3.9.0-preview.5757 to 3.9.0, and ElsaStudioVersion from 3.9.0-preview.1797 to 3.9.0.

Verified

• At this head, every Elsa.* and Elsa.Studio.* entry in Directory.Packages.props uses $(ElsaVersion) or $(ElsaStudioVersion). The only other Elsa-named entry is Elsa.Platform.PackageManifest.Generator (0.0.1-preview.50). It's a separate tool with its own version, and this release doesn't need to change it.
• All 44 distinct packages that use those two properties resolve on nuget.org at 3.9.0; I checked each .nuspec and none is missing.
• CI is green at this head: ubuntu-latest (4m42s), submit-nuget (1m43s), GitGuardian and the CLA check. So the build and tests pass against the released 3.9.0 packages.

Gate: APPROVE + HIGH with green CI. Greptile is waived for this repo.

No blocking or non-blocking notes.