Update browser baseline for boot script

[robertknight]

The client's boot script supports older browsers than the rest of the client so it can log a warning if it can't start in "somewhat" older browsers.

Update the boot script's supported officially browsers from ~2013 (IE 11) to ~2017 (Safari 11), although the code will probably still run in older browsers. This allows dropping an async/await Babel plugin, although async is not actually used in the current boot script.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.43%. Comparing base (1b5c407) to head (bf80afa). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #6654 +/- ## ======================================= Coverage 99.43% 99.43% ======================================= Files 271 271 Lines 10198 10198 Branches 2423 2423 ======================================= Hits 10140 10140 Misses 58 58 ```

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