MM-59827: Remove user profile invalidations from channel APIs (#27720)

We were invalidating the userProfileByIds cache unnecessarily
when it wasn't really needed. It's just that the InvalidateCacheForUser
method wiped off all user related caches, when we just needed to wipe
user-channel related caches.

To fix this, we split the user cache invalidation method into two
and just call the InvalidateChannelCacheForUser method from app/channel.go.

https://mattermost.atlassian.net/browse/MM-59827
```release-note
NONE
```

server/channels/app/platform/web_hub.go

@@ -196,11 +196,15 @@ func (ps *PlatformService) InvalidateCacheForChannelPosts(channelID string) {
 }
 
 func (ps *PlatformService) InvalidateCacheForUser(userID string) {
+	ps.InvalidateChannelCacheForUser(userID)
+	ps.Store.User().InvalidateProfileCacheForUser(userID)
+}
+
+func (ps *PlatformService) InvalidateChannelCacheForUser(userID string) {
 	ps.Store.Channel().InvalidateAllChannelMembersForUser(userID)
 	ps.ClearUserSessionCache(userID)
 
 	ps.Store.User().InvalidateProfilesInChannelCacheByUser(userID)
-	ps.Store.User().InvalidateProfileCacheForUser(userID)
 }
 
 func (ps *PlatformService) InvalidateCacheForUserTeams(userID string) {