caught-up: Drop legacy inference for servers pre-1.8

Huzzah!  Look at that negative diffstat.

Also make a note next to where we record `minSupportedVersion`, in
ServerCompatBanner.js.  We may at some point want to have the app
outright refuse to operate on sufficiently ancient servers, where
it's just not going to be a good experience.  Until then, it seems
helpful to track what we know about what the consequences are
likely to be (though actually *testing* on such ancient servers
would be significantly more effort and not worth it.)

Author: gnprice

src/__tests__/lib/exampleData.js

@@ -713,8 +713,8 @@ export const action = Object.freeze({
     anchor: 0,
     numBefore: 50,
     numAfter: 50,
-    foundNewest: undefined,
-    foundOldest: undefined,
+    foundNewest: false,
+    foundOldest: false,
     ownUserId: selfUser.user_id,
   }): MessageFetchCompleteAction),
   // If a given action is only relevant to a single test file, no need to

src/actionTypes.js

@@ -221,8 +221,8 @@ export type MessageFetchCompleteAction = {|
   anchor: number,
   numBefore: number,
   numAfter: number,
-  foundNewest: boolean | void,
-  foundOldest: boolean | void,
+  foundNewest: boolean,
+  foundOldest: boolean,
   ownUserId: UserId,
 |};
 

src/api/messages/getMessages.js

@@ -10,9 +10,9 @@ import { AvatarURL } from '../../utils/avatar';
 type ApiResponseMessages = {|
   ...$Exact<ApiResponseSuccess>,
   anchor: number,
-  found_anchor?: boolean,
-  found_newest?: boolean,
-  found_oldest?: boolean,
+  found_anchor: boolean,
+  found_newest: boolean,
+  found_oldest: boolean,
   messages: Message[],
 |};
 
@@ -84,13 +84,7 @@ const migrateResponse = (response, identity: Identity) => {
 
 /**
  * See https://zulip.com/api/get-messages
- *
- * These values exist only in Zulip 1.8 or newer:
- *   * found_anchor
- *   * found_newest
- *   * found_oldest
  */
-// TODO(server-1.8): Mark those properties as required; simplify downstream.
 export default async (
   auth: Auth,
   args: {|