Rename workflows to make it clearer (#23316)

Mattermost build was a confusingly named workflow. Changed it
to Server CI to make it consistent with Webapp CI.

Also changed the filenames.

```release-note
NONE
```

.github/workflows/artifacts.yml

@@ -1,7 +1,7 @@
 name: Artifacts generation and upload
 on:
   workflow_run:
-    workflows: ["Mattermost Build"]
+    workflows: ["Server CI"]
     types:
       - completed
 

.github/workflows/server-ci.yml

@@ -1,4 +1,4 @@
-name: Mattermost Build
+name: Server CI
 on:
   pull_request:
   push: