chore: specify an explicit browserslist version

[pauldambra]

pulling small changes out of #1525 because IE 11 tests are freezing in that PR and it's really hard to figure out why

in that PR I added an explicit browserslist version, let's do that separately

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Nov 19, 2024 11:13am

[github-actions]

Size Change: 0 B

Total Size: 3.11 MB

ℹ️ View Unchanged

Filename	Size
dist/all-external-dependencies.js	204 kB
dist/array.full.es5.js	253 kB
dist/array.full.js	356 kB
dist/array.full.no-external.js	355 kB
dist/array.js	170 kB
dist/array.no-external.js	169 kB
dist/dead-clicks-autocapture.js	13.6 kB
dist/exception-autocapture.js	8.7 kB
dist/external-scripts-loader.js	2.19 kB
dist/main.js	171 kB
dist/module.full.js	356 kB
dist/module.full.no-external.js	355 kB
dist/module.js	170 kB
dist/module.no-external.js	169 kB
dist/recorder-v2.js	113 kB
dist/recorder.js	113 kB
dist/surveys-preview.js	56.6 kB
dist/surveys.js	62.1 kB
dist/tracing-headers.js	1.33 kB
dist/web-vitals.js	10.3 kB

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[robbie-c]

Looks like specifying an explicit browserslist version to me!