Suppress browserslist warning in webpack target (go-gitea#30571)

1. Set
[`BROWSERSLIST_IGNORE_OLD_DATA`](https://github.com/browserslist/browserslist/blob/c6ddf7b3870a4585822d06ec77e8dd2401b8e1ed/node.js#L400)
to avoid warning on outdated browserslist data which the end user can
likely not do anything about and which is currently visible in the v1.21
branch.
2. Suppress all command echoing and add a "Running webpack..." message
in place.

Warning in question was this:

```
Browserslist: caniuse-lite is outdated. Please run:
  npx update-browserslist-db@latest
  Why you should do it regularly: https://github.com/browserslist/update-db#readme
```

Makefile

@@ -908,8 +908,9 @@ webpack: $(WEBPACK_DEST)
 
 $(WEBPACK_DEST): $(WEBPACK_SOURCES) $(WEBPACK_CONFIGS) package-lock.json
 	@$(MAKE) -s node-check node_modules
-	rm -rf $(WEBPACK_DEST_ENTRIES)
-	npx webpack
+	@rm -rf $(WEBPACK_DEST_ENTRIES)
+	@echo "Running webpack..."
+	@BROWSERSLIST_IGNORE_OLD_DATA=true npx webpack
 	@touch $(WEBPACK_DEST)
 
 .PHONY: svg