[api-merge] Add removed-since info (#8897)

Our process of building `Mono.Android.dll`'s `api.xml` using
`api-merge` does not take into consideration that API could be
*removed* from `android.jar`.

Track API removals by adding a `removed-since="API-LEVEL"` attribute
to API members that no longer exists.

Note this information is not currently consumed anywhere, but having
this data will allow us to determine if we want to make modifications
to handle it in future PRs.  Potentially
[`[UnsupportedOSPlatformAttribute]`][0] is a good solution for this.

[0]: https://learn.microsoft.com/dotnet/api/system.runtime.versioning.unsupportedosplatformattribute?view=net-8.0

build-tools/api-merge/ApiDescription.cs

@@ -121,6 +121,13 @@ public void Merge (XDocument n, string apiLocation)
 						}
 					}
 #endif
+					foreach (var smember in stype.Elements ()) {
+						var nmember = GetMember (ntype, smember);
+
+						if (nmember is null)
+							SetRemovedSince (smember, platform);
+					}
+
 					foreach (var nmember in ntype.Elements ()) {
 						var smember = GetMember (stype, nmember);
 						if (smember == null) {
@@ -308,6 +315,16 @@ void SetDeprecatedSince (XElement element, string platform)
 				deprecatedSince.SetValue (platform);
 		}
 
+		void SetRemovedSince (XElement element, string platform)
+		{
+			if (element is null)
+				return;
+
+			// Don't replace an earlier removal, as we want to keep the earliest one.
+			if (!element.Attributes ("removed-since").Any ())
+				element.Add (new XAttribute ("removed-since", platform));
+		}
+
 		XElement AddWithLocation (XElement old, XElement child, string location)
 		{
 			child.Add (new XAttribute ("merge.SourceFile", location));