Open 10.4.0 on release/10.x #473

Change

Open 10.4.0 on release/10.x right after v10.3.0 (#470, step 6), as #426 did for 10.3.0.

1. Directory.Build.props: Version 10.4.0, and PackageValidationBaselineVersion 10.3.0, the release being superseded.
2. SECURITY.md: the supported row becomes 10.3.x and the unsupported row < 10.3. SecurityPolicyConventionTests holds the row to the baseline.
3. samples/Bookings/Bookings.csproj: the package version 10.3.0, which moves with the baseline (WalkthroughSampleConventionTests).
4. Changelogs: an undated ## [10.4.0] section in the root changelog and in each package changelog. Each package section says "No change in this package." until a PR replaces it.

Done when

The PR is merged into release/10.x, and the conventions suite passes.