Refactor check of Coursier ordering onto Version.Update

This change just refactors the logic of the check to live on `Version.Update`
(introduced with 13380eb in PR #3145 in August 2023), which feels like a
good place for it to belong.

The check for whether our version-updates comply with Coursier
version-ordering was added with this PR in January 2020:

* #1210

...the check was supposed to be temporary, until Scala Steward's
own `Order[Version]` was considered battle-tested, but I don't know
when we'll decide that's happened!

## `current == next` is ok?

Since commit 75c08e6, committed directly to `main` a day later,
`VersionOrderingConflict` has only been raised if `current > next` - so
`current == next` is fine. There's no further info on _why_ in the commit
tho'!

75c08e6

## New `coursier-versions` library

Our Scala Steward check uses `coursier.core.Version`, which is deprecated since
Coursier 2.1.25:

https://github.com/coursier/coursier/blame/0f3ceb65e9e46826967d2e4cb6f87cf7fb3e5c9d/modules/core/shared/src/main/scala/coursier/core/Version.scala#L14

...the recommended class is now `coursier.version.Version`, which is in the newer
https://github.com/coursier/versions library created in May 2020:

https://github.com/coursier/versions/blob/main/versions/shared/src/coursier/version/Version.scala

Author: rtyley

modules/core/src/main/scala/org/scalasteward/core/data/Version.scala

@@ -122,7 +122,10 @@ final case class Version(value: String) {
 }
 
 object Version {
-  case class Update(currentVersion: Version, nextVersion: Version)
+  case class Update(currentVersion: Version, nextVersion: Version) {
+    def obeysCoursierOrdering: Boolean =
+      coursier.core.Version(nextVersion.value) >= coursier.core.Version(currentVersion.value)
+  }
 
   def show(versions: Version*): String = {
     val vs0 = versions.map(_.value)

modules/core/src/main/scala/org/scalasteward/core/update/FilterAlg.scala

@@ -129,10 +129,7 @@ object FilterAlg {
   }
 
   private def checkVersionOrdering(
-      update: Update.ForArtifactId
-  ): Either[RejectionReason, Update.ForArtifactId] = {
-    val current = coursier.core.Version(update.currentVersion.value)
-    val next = coursier.core.Version(update.nextVersion.value)
-    if (current > next) Left(VersionOrderingConflict(update)) else Right(update)
-  }
+      u: Update.ForArtifactId
+  ): Either[RejectionReason, Update.ForArtifactId] =
+    Either.cond(u.versionUpdate.obeysCoursierOrdering, u, VersionOrderingConflict(u))
 }

modules/core/src/test/scala/org/scalasteward/core/data/VersionTest.scala

@@ -5,12 +5,21 @@ import cats.kernel.laws.discipline.OrderTests
 import munit.DisciplineSuite
 import org.scalacheck.Prop.*
 import org.scalasteward.core.TestInstances.*
+import org.scalasteward.core.TestSyntax.StringOps
 import org.scalasteward.core.data.Version.Component
+
 import scala.util.Random
 
 class VersionTest extends DisciplineSuite {
   checkAll("Order[Version]", OrderTests[Version].order)
 
+  test("Note that currently, we treat Snapshot vs Milestone differently to Coursier") {
+    val snapshot = "1.0.0-SNAP8"
+    val milestone = "1.0.0-M2"
+    assert(snapshot.v < milestone.v)
+    assert(coursier.core.Version(snapshot) > coursier.core.Version(milestone)) // Surprising to me
+  }
+
   test("issue 1615: broken transitivity") {
     val res = OrderTests[Version].laws.transitivity(Version(""), Version("0"), Version("X"))
     assertEquals(res.lhs, res.rhs)