Revert "Drop iOS 14 and macOS 11 support"

This reverts commit dcef7f47ff.

macOS/InstancesSettings.swift

@@ -38,7 +38,7 @@ struct InstancesSettings: View {
             if !selectedInstance.isNil, selectedInstance.app.supportsAccounts {
                 SettingsHeader(text: "Accounts".localized())
 
-                List(selection: $selectedAccount) {
+                let list = List(selection: $selectedAccount) {
                     if selectedInstanceAccounts.isEmpty {
                         Text("You have no accounts for this location")
                             .foregroundColor(.secondary)
@@ -69,7 +69,13 @@ struct InstancesSettings: View {
                         .tag(account)
                     }
                 }
-                .listStyle(.inset(alternatesRowBackgrounds: true))
+
+                if #available(macOS 12.0, *) {
+                    list
+                        .listStyle(.inset(alternatesRowBackgrounds: true))
+                } else {
+                    list
+                }
             }
 
             if selectedInstance != nil, selectedInstance.app.hasFrontendURL {