[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

Author: jpobst

build-tools/api-merge/ApiDescription.cs

@@ -121,6 +121,13 @@ public void Merge (XDocument n, string apiLocation)
 }
 }
 #endif
+foreach(varsmemberinstype.Elements()){
+varnmember=GetMember(ntype,smember);
+
+if(nmemberisnull)
+SetRemovedSince(smember,platform);
+}
+
 foreach(varnmemberinntype.Elements()){
 varsmember=GetMember(stype,nmember);
 if(smember==null){
@@ -308,6 +315,16 @@ void SetDeprecatedSince (XElement element, string platform)
 deprecatedSince.SetValue(platform);
 }
 
+voidSetRemovedSince(XElementelement,stringplatform)
+{
+if(elementisnull)
+return;
+
+// Don't replace an earlier removal, as we want to keep the earliest one.
+if(!element.Attributes("removed-since").Any())
+element.Add(newXAttribute("removed-since",platform));
+}
+
 XElementAddWithLocation(XElementold,XElementchild,stringlocation)
 {
 child.Add(newXAttribute("merge.SourceFile",location));