fix: reduce keep alive threshold

[pauldambra]

follow-up to #1416

I still sometimes see TypeError: Failed to fetch

we check if the request is less than 64kb, but maybe we then add overhead and so fetch rejects it

let's reduce the threshold so that we can't possibly trigger this error for fetch

[vercel]

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Oct 28, 2024 3:19pm

[github-actions]

Size Change: +20 B (0%)

Total Size: 2.94 MB

Filename	Size	Change
dist/array.full.es5.js	249 kB	+2 B (0%)
dist/array.full.js	335 kB	+2 B (0%)
dist/array.full.no-external.js	334 kB	+2 B (0%)
dist/array.js	166 kB	+2 B (0%)
dist/array.no-external.js	165 kB	+2 B (0%)
dist/main.js	167 kB	+2 B (0%)
dist/module.full.js	335 kB	+2 B (0%)
dist/module.full.no-external.js	334 kB	+2 B (0%)
dist/module.js	166 kB	+2 B (0%)
dist/module.no-external.js	165 kB	+2 B (0%)

ℹ️ View Unchanged

Filename	Size
dist/all-external-dependencies.js	182 kB
dist/exception-autocapture.js	8.77 kB
dist/external-scripts-loader.js	2.19 kB
dist/recorder-v2.js	102 kB
dist/recorder.js	103 kB
dist/surveys-preview.js	56.7 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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