[2.x] fix: Exclude autoStartServer from lintUnused warnings (#6624) (#8708)

**Problem**
Setting `Global / autoStartServer := false` in global.sbt triggers a
spurious lintUnused warning, even though the setting is correctly used
by sbt's server startup logic.

**Solution**
Add `autoStartServer` to the `excludeLintKeys` set in LintUnused.scala,
similar to other server-related settings like `serverConnectionType` and
`serverIdleTimeout`. This prevents the warning while maintaining the
functionality of the setting.

main/src/main/scala/sbt/internal/LintUnused.scala

@@ -29,6 +29,7 @@ object LintUnused {
     },
     excludeLintKeys := Set(
       aggregate,
+      autoStartServer,
       concurrentRestrictions,
       commands,
       configuration,

sbt-app/src/sbt-test/project/lint-autostartserver/build.sbt

@@ -0,0 +1,23 @@
+// Test for issue #6624: autoStartServer should not trigger lintUnused warning
+Global / autoStartServer := false
+
+@transient
+lazy val check = taskKey[Unit]("")
+
+lazy val root = (project in file("."))
+  .settings(
+    check := Def.uncached {
+      val state = Keys.state.value
+      val includeKeys = (Global / lintIncludeFilter).value
+      val excludeKeys = (Global / lintExcludeFilter).value
+      val result = sbt.internal.LintUnused.lintUnused(state, includeKeys, excludeKeys)
+      // autoStartServer should not appear in the lint results
+      val autoStartServerWarnings = result.filter { case (_, key, _) =>
+        key.contains("autoStartServer")
+      }
+      assert(
+        autoStartServerWarnings.isEmpty,
+        s"autoStartServer should not trigger lintUnused warnings, but found: ${autoStartServerWarnings.map(_._2).mkString(", ")}"
+      )
+    }
+  )

sbt-app/src/sbt-test/project/lint-autostartserver/test

@@ -0,0 +1 @@
+> check