Slick v4 migration - #825

This PR is a bit early, but I'm working with releasing Slick 4 and would like to present this PR to show the kind of chanages needed to downstream libraries extending Slick (which I don't know many). The version referred to by the PR does not exist publicly. I will update the PR once it does.

This PR migrates slick-pg to Slick v4. The changes are straightforward and mostly limited to how tests obtain a database connection — the library code itself has only minor updates.

Changes

build.sbt

• Slick updated from 3.6.1 to 4.0.0
• Added slick-future, cats-effect, and fs2-core dependencies (required by the slick-future module)
• Scala 3 updated from 3.3.1 to 3.8.3; cross-compilation retained for 2.12, 2.13 and 3
• play-json dependency simplified: the Scala-version conditional is no longer needed since org.playframework supports all three versions

Core library

• ExPostgresProfile: createModelBuilder and ExModelBuilder no longer take an implicit ExecutionContext parameter, which was removed from the Slick v4 signature.
• lobj/LargeObjectStreamingDBIOAction: Replaced the Slick v3 emitStream/cancelStream pair (along with StreamState, implicit proof, and the 5-type-parameter SynchronousDatabaseAction) with the new Slick v4 openStream returning a CloseableIterator[Array[Byte]].
• date/PgDateJdbcTypes: The mapTo helper's explicit implicit argument passing (ctag, this) to MappedJdbcType.base was replaced with implicit val self brought into scope. This is required for Scala 3 compatibility, where context bounds map to using parameters and can no longer be satisfied with explicit argument syntax.

Test infrastructure

• PostgresContainer: db is no longer a lazy val defined per test class. It is now a lifecycle-managed shared resource — opened in afterStart() using slick.future.Database (from the new slick-future module) and closed in beforeStop(). Test suites continue to use Await.result(db.run(...), Duration.Inf) unchanged.
• ScalaVersionShim (scala-2 / scala-3): WeekDay type converters for PgCompositeSupportSuite moved from an inline Scala-2-only block into version-specific shims. This is required for Scala 3 compatibility, where register() relies on implicit resolution and cannot be called with the same syntax as in Scala 2.

All test suites

Uniform mechanical change across every test suite: removed lazy val db = Database.forurl(/sitelet?url=https%3A%2F%2Fgithub.com%2Ftminglei%2Fslick-pg%2Fpull%2F...), which is now provided by PostgresContainer. All other test code is unchanged — Await.result(db.run(...), Duration.Inf) continues to work as before thanks to slick.future.Database providing the same run/stream API as Slick 3.