Open 10.2.0 on release/10.x #331

Change

Open the 10.x version after 10.1.0 on release/10.x, as #304 did for 10.1.0: the first PR of that release, before any fix. Whoever picks it up chooses the number:

• A minor (10.2.0) if the release carries a silent fix or an addition (the additions window is set in CONTRIBUTING.md).
• A patch (10.1.1) if it carries only non-silent fixes, such as 10.x: the empty-root rewrite casts EF Core's _ord column to the range type on EF Core 10.0.2 and later (port of #319) #325 or 10.x: RangeSetContains.Equivalent hands any other one-argument Contains to the value overload (port of #320) #326.

1. Directory.Build.props: set Version to the new version and PackageValidationBaselineVersion to 10.1.0, the release being superseded.
2. SECURITY.md: the supported row becomes 10.1.x and the unsupported row < 10.1, naming the latest release on nuget.org, never the version being prepared. SecurityPolicyConventionTests compares the row with the baseline, so the row can only move together with step 1. This condition was moved here from Release 10.1.0: open the version on release/10.x, date the changelogs and tag v10.1.0 #301's done-when on 2026-10-19. main already moved its own table in chore: carry the 10.1.0 release to main #330.
3. Changelogs: a ## [<version>] section, undated, in the root changelog and in each package changelog. Each package section says "No change in this package." until a fix PR replaces it.

Then create the release epic and milestone for that version, and move the issues it will ship under it (from backlog #83).

Done when

The PR is merged into release/10.x, and SecurityPolicyConventionTests, ChangelogConventionTests and PackagingConventionTests pass.