Keep Capability experimental

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -2098,6 +2098,7 @@ class Definitions {
    */
   @tu lazy val ccExperimental: Set[Symbol] = Set(
     CapsModule, CapsModule.moduleClass, PureClass,
+    Caps_Capability, // TODO: Remove when Capability is stabilized
     RequiresCapabilityAnnot,
     captureRoot, Caps_CapSet, Caps_ContainsTrait, Caps_ContainsModule, Caps_ContainsModule.moduleClass, UseAnnot,
     Caps_Mutable, Caps_SharedCapability, ConsumeAnnot,

library/src/scala/caps/package.scala

@@ -22,6 +22,7 @@ import annotation.{experimental, compileTimeOnly, retainsCap}
  * But even without capture checking, extending this trait can be useful for documenting the intended purpose
  * of a class.
  */
+@experimental
 trait Capability extends Any
 
 /** The universal capture reference. */
@@ -89,6 +90,7 @@ final class consume extends annotation.StaticAnnotation
 /** A trait that used to allow expressing existential types. Replaced by
 *  root.Result instances.
 */
+@experimental
 @deprecated
 sealed trait Exists extends Capability