docs(dev-infra): add instructions for updating GeckoDriver in `browsers/README.md` (#40150)
docs(dev-infra): add instructions for updating GeckoDriver in `browsers/README.md` (#40150)

This commit adds instructions in `dev-infra/browsers/README.md` on how
to update the version of GeckoDriver (the WebDriver implementation for
Firefox browsers).

NOTE:
The gecko driver download URLs perform redirection, so the `curl`
command needs to be run with the `-L/--location` option to follow
redirects. I updated all `curl` commands (even those that are not used
on gecko driver download URLs) for consistency.

PR Close #40150

angular/dev-infra,angular/dev-infra,angular/dev-infra,angular/dev-infra