chore: remove leftover .NET traces from CI and docker context

[cyberantonz]

The .NET identity service is fully decommissioned and no C# code remains in the repository, but two pieces of tooling still assumed it existed:

• semgrep workflow: drop the p/csharp ruleset from both scan passes (nothing left for it to match) and update the comment/summary text accordingly.
• src/backend/.dockerignore: remove the .NET bin/obj block, Visual Studio state (.vs, *.user, *.suo), and TestResults, and fix the stale header that still described the build context as serving the retired api-gateway/analytics/identity trio.

Deliberately not touched (load-bearing history/parity documentation): the identity-resolution migration parity comments and frozen sql/001–013 scripts, the e2e capability shims in tests/e2e/lib/identity.py, and historical mentions in specs/ADRs.

Known remaining .NET residue, out of scope here: the committed identity OpenAPI contract is still the .NET document (blocked on identity-resolution growing an openapi subcommand, as authenticator did in #2000), and migration/mod.rs still states the additive-only-until-decommission rollback policy.

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📥 Commits

Reviewing files that changed from the base of the PR and between 27eac0e and 4e69d92.

📒 Files selected for processing (2)

• .github/workflows/semgrep.yml
• src/backend/.dockerignore

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