chore: hold the package version in one place

[andregoepel]

The per-project <Version> fields in this repo were dead and misleading.

CI publishes with -p:Version=${GITHUB_REF_NAME#v}, so the git tag has
always been the real version. These values never reached NuGet — they only told
anyone reading the csproj, or packing locally, something untrue.

	Value
csproj said	1.0.0
actually published	1.0.2

The change

One <Version> in the shared Directory.Build.props — which every package
here already inherits for Authors, RepositoryUrl and the rest — and the
per-project copies removed. Seeded with the version actually published.

Consolidated rather than just corrected, because correcting would drift again on
the next release. app-foundation is the proof of that: six copies, five reading
1.8.0 and MailService reading 1.9.0, against a published 2.1.0 — per-project
versioning was once intended and died quietly when CI started packing everything
from one tag. This makes that reality explicit instead of leaving six places to
forget.

Behaviour is unchanged where it matters

Verified by packing both ways rather than reasoning about it:

	Produces
CI path — dotnet pack -p:Version=9.9.9	…9.9.9.nupkg
local path — no override	…1.0.2.nupkg (previously a stale number)

-p:Version still wins, so nothing about publishing changes.

Verification

• dotnet build — 0 errors, 0 warnings
• Full test suite green
• dotnet csharpier check . clean
• Only Directory.Build.props and src/*/*.csproj touched

Part of a sweep across all seven packable repos, which were stale in five of
them.