Update mattermost library (#2152)

* Update mattermost library

* Fix linting

vendor/github.com/hashicorp/golang-lru/README.md

@@ -1,25 +1,7 @@
 golang-lru
 ==========
 
-This provides the `lru` package which implements a fixed-size
-thread safe LRU cache. It is based on the cache in Groupcache.
-
-Documentation
-=============
-
-Full docs are available on [Godoc](https://pkg.go.dev/github.com/hashicorp/golang-lru)
-
-Example
-=======
-
-Using the LRU is very simple:
-
-```go
-l, _ := New(128)
-for i := 0; i < 256; i++ {
-    l.Add(i, nil)
-}
-if l.Len() != 128 {
-    panic(fmt.Sprintf("bad len: %v", l.Len()))
-}
-```
+Please upgrade to github.com/hashicorp/golang-lru/v2 for all new code as v1 will
+not be updated anymore. The v2 version supports generics and is faster; old code
+can specify a specific tag, e.g. github.com/hashicorp/golang-lru/v1.0.2 for
+backwards compatibility.