browserslist: Stop transpiling for pre-ES2019 browsers.

Specifically, this desupports:

android 4.4.3-4.4.4
baidu 7.12
ie 11
kaios 2.5
op_mini all

although we’ve already been blocking IE 11 since 3.0 (#14662).

Signed-off-by: Anders Kaseorg <anders@zulip.com>

.browserslistrc

@@ -2,4 +2,4 @@
 > 0.15% in US
 last 2 versions
 Firefox ESR
-not dead
+not dead and supports async-functions

babel.config.js

@@ -15,7 +15,6 @@ module.exports = {
             "@babel/preset-env",
             {
                 corejs: "3.20",
-                loose: true, // Loose mode for…of loops are 5× faster in Firefox
                 shippedProposals: true,
                 useBuiltIns: "usage",
             },

tools/ci/production-build

@@ -44,9 +44,9 @@ cp -a \
     \
     /tmp/production-build
 
-# Check that webpack bundles use only ES5 syntax.
+# Check that webpack bundles use only ES2019 syntax.
 tar -C /tmp -xzf /tmp/production-build/zulip-server-test.tar.gz zulip-server-test/prod-static/serve/webpack-bundles
 (
     GLOBIGNORE=/tmp/zulip-server-test/prod-static/serve/webpack-bundles/katex-cli.js
-    yarn run es-check es5 /tmp/zulip-server-test/prod-static/serve/webpack-bundles/*.js
+    yarn run es-check es2019 /tmp/zulip-server-test/prod-static/serve/webpack-bundles/*.js
 )