Bump Serilog.Sinks.File 6 → 7 (held with the Serilog sink ecosystem)

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Context

Dependabot PR #29 bumped Serilog.Sinks.File from 6.0.0 to 7.0.0.
The major bump is held back in commit 9bada36 alongside the
Tomlyn / System.CommandLine deferrals so the routine NuGet
maintenance cycle doesn't conflate unrelated migrations.

Acceptance

• Serilog.Sinks.File pinned at 7.x (or current latest).
• All log-related smoke tests stay green; verify rolling file
  sink behaviour still rotates as expected.
• If the rest of the Serilog stack has follow-up majors at the
  same time, fold those in.