chore: bump server version to 0.3.0 (#7624)

This PR bumps the server version so that clients like NeoVim can detect
whether the server supports our recent language server extensions
(modulo the time that has passed since these extension PRs).

I'd like to have server capabilities for this at some point, but this
will have to do for now.

src/Lean/Server/Watchdog.lean

@@ -1296,7 +1296,7 @@ def initAndRunWatchdog (args : List String) (i o e : FS.Stream) : IO Unit := do
       capabilities := mkLeanServerCapabilities
       serverInfo?  := some {
         name     := "Lean 4 Server"
-        version? := "0.2.0"
+        version? := "0.3.0"
       }
       : InitializeResult
     }