fix(migrate): bump Nuke→Fallout PackageReference versions + strip stale Cryptography.Xml pin (#217)

[ChrisonSimtian]

Summary

Closes #217, reported by @dennisdoomen while migrating FluentAssertions to Fallout.

After `fallout-migrate`, the generated `_build.csproj` failed to restore with:

```
error NU1603: Warning As Error: _build depends on Fallout.Common (>= 10.1.0) but Fallout.Common 10.1.0 was not found. Fallout.Common 10.2.12 was resolved instead.
error NU1605: Warning As Error: Detected package downgrade: System.Security.Cryptography.Xml from 10.0.6 to 9.0.15.
```

Two root causes, both fixed here:

1. NUKE-era `Version="10.1.0"` pins survived the namespace rewrite. `fallout-migrate` only ran a `Nuke.X` → `Fallout.X` Include attribute swap; the `Version="..."` attribute on the same `` element was untouched. Result: `` in the output — but `Fallout.Common 10.1.0` was never published (oldest is `10.2.12`). NuGet's NU1603 fallback + `WarningsAsErrors` in the migrated csproj escalated to fatal.
2. Stale `System.Security.Cryptography.Xml` direct pin. NUKE-era projects often pinned an older major (9.x). Fallout.Common's transitive requirement is `≥ 10.0.6`, so the explicit pin produces NU1605 ("Detected package downgrade"). Dennis's manual workaround was to remove the pin entirely.

What this PR changes

`src/Fallout.Migrate/CsprojRewriter.cs` — two new regex passes:

• `NukePackageWithInlineVersionPattern` — combined rewrite for `<PackageReference Include="Nuke.X" ... Version="Y" />`. Tolerates extra attrs like `PrivateAssets="all"` between Include and Version. Replaces both the Include namespace AND the Version pin to the running migrate tool's own version.
• `CryptographyXmlPackageRefPattern` — strips explicit `System.Security.Cryptography.Xml` PackageReferences entirely. Lets Fallout.Common's transitive version win.

Existing namespace-only `PackageReferencePattern` still handles CPM-managed references (no inline Version — version lives in `Directory.Packages.props`), so we don't accidentally inject a Version where there wasn't one.

`src/Fallout.Migrate/Migration.cs` — `RewriteCsprojs` derives `falloutVersion` from `AssemblyInformationalVersionAttribute` (stripping the `+gabc...` build-metadata suffix). Inlined to keep `Fallout.Migrate` dependency-free (it has no ProjectReferences today). Falls back to a hardcoded `11.0.0` for local dev builds.

`tests/Fallout.Migrate.Tests/CsprojRewriterTest.cs` — 4 existing tests updated to pass the new `falloutVersion` parameter (no behavioural change for them) + 5 new tests covering:

• `BumpsNukeInlineVersionToCurrentFalloutVersion` — direct regression guard for Fallout-migrate triggers some dependency confusion #217.
• `BumpsVersionAcrossExtraAttributesBetweenIncludeAndVersion` — `PrivateAssets="all"` between Include and Version still works.
• `LeavesCpmManagedReferencesWithoutInlineVersionUntouchedByVersionPass` — no Version-injection where the user didn't have one.
• `StripsSystemSecurityCryptographyXmlPackageReference` — the second sub-bug.
• `LeavesOtherSystemPackagesAlone` — only Cryptography.Xml is targeted; other System.* are not touched.

`CHANGELOG.md` — entry added under `[Unreleased] — 11.0` alongside existing bug-fix bullets.

Test plan

• CI green (Fallout.Migrate.Tests: 22/22 locally; was 17/17 before).
• After merge, Dennis re-runs `fallout-migrate` on FluentAssertions and the migrated `_build.csproj` restores without NU1603 / NU1605.

Refs

• Closes Fallout-migrate triggers some dependency confusion #217. The second bug from the same session (GitVersion parsing fails with error #218 GitVersion JSON parsing) is a separate fix surface and stays open for a follow-up PR.