fix(plugin-postgresql): rename versioned capabilities to avoid name collision

[datlechin]

Summary

Main is broken: PostgreSQLPluginDriver declares two properties named capabilities — PostgreSQLCapabilities (from PR #1241) and PluginCapabilities (from the protocol). The compiler can't disambiguate at the call sites in PostgreSQLPluginDriver+Columns.swift.

Also fixes a stray majorVersion reference left over from the helper that PR #1241 removed.

Changes

• Rename var capabilities: PostgreSQLCapabilities → var versionedCapabilities. Updates all call sites in PostgreSQLPluginDriver.swift and PostgreSQLPluginDriver+Columns.swift.
• Add PostgreSQLCapabilities.hasModernICUSyntax (PG 16+) and replace the dangling majorVersion check in CREATE DATABASE ICU locale branch with versionedCapabilities.hasModernICUSyntax.

Why two properties named capabilities?

PluginCapabilities is an OptionSet declared in the plugin protocol (alterTableDDL, materializedViews, etc.). Each plugin exposes the set it supports. PostgreSQLCapabilities from #1241 is per-server-version (gates against pg_matviews, attidentity, etc.). The two are unrelated; same name was a collision, not a design choice.

versionedCapabilities makes the distinction explicit.

Test plan

• xcodebuild build — green (was failing on main).