Build: pin SDK-implicit FSharp.Core and bump FsharpVersion to 10.1.302 (fixes repo-wide NU1605)

[Aaronontheweb]

Fixes repo-wide CI break: NU1605 FSharp.Core downgrade

Every CI run since ~2026-07-14 fails at compile with:

error NU1605: Warning As Error: Detected package downgrade: FSharp.Core from 10.1.302 to 10.1.301.
 Akka.FSharp.Tests -> Akka.FSharp -> FSharp.Core (>= 10.1.302)
 Akka.FSharp.Tests -> FSharp.Core (>= 10.1.301)

Root cause

Akka.FSharp had no explicit FSharp.Core reference, so it rode the F# SDK's implicit one — whose version tracks whatever .NET SDK is installed on the build agent. The Azure hosted images rolled a newer SDK between the last green dev build (2026-07-12) and 2026-07-15, bumping the implicit FSharp.Core to 10.1.302, which now conflicts with the repo-pinned $(FsharpVersion) of 10.1.301 used by Akka.FSharp.Tests. This is infrastructure drift — no code change in this repo triggered it, and it breaks every branch and PR identically (all four compile jobs fail with the same error).

Fix (2 lines)

1. Bump <FsharpVersion> 10.1.301 → 10.1.302 in Directory.Build.props.
2. Pin the SDK-implicit reference in Akka.FSharp.fsproj via <PackageReference Update="FSharp.Core" Version="$(FsharpVersion)"/> — the same Update technique Akka.FSharp.Tests already uses. This makes the resolved FSharp.Core independent of the build agent's SDK, so future image rolls can't reintroduce NU1605 from either direction (this incident was SDK-ahead-of-pin; the pre-existing comment in the test project records the earlier FsCheck-ahead-of-SDK incident). It also makes the published Akka.FSharp package's FSharp.Core dependency floor a deliberate, reviewable value instead of an artifact of whichever SDK image packed the release.

Verification

• Restore/build clean locally; both projects resolve FSharp.Core to exactly 10.1.302 in project.assets.json
• Akka.FSharp.Tests: 35 passed / 1 skipped
• This same commit (cherry-picked from Akka.Serialization.V2: closed generic messages + manifest-discriminated union fields (PoC for #8384) #8385, where CI first hit the break) took that PR's validation run from four failed compile jobs to fully green across all 11 checks