Add browserslist and improve CSS masks for compatibility

[conico974]

Introduce a browserslist configuration to support a wider range of browsers and enhance CSS mask properties for better compatibility with older browsers.

This configuration supports 93.3% of the browser worldwide

[linear]

RND-7471 Certain css properties not supported on all browsers

[changeset-bot]

🦋 Changeset detected

Latest commit: 06a56b9

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
gitbook	Patch
@gitbook/icons	Patch
@gitbook/react-contentkit	Patch

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Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

Summary of the deployments:

Version	URL	Status
Vercel	https://gitbook-v2-3862b4ggl-gitbook.vercel.app	✅
Cloudflare	https://6f74b350-gitbook-open-v2-preview.gitbook.workers.dev	✅

Test content

Site	2v	2c
GitBook	https://gitbook-v2-3862b4ggl-gitbook.vercel.app/url/gitbook.com/docs	https://6f74b350-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.com/docs
E2E	https://gitbook-v2-3862b4ggl-gitbook.vercel.app/url/gitbook.gitbook.io/test-gitbook-open	https://6f74b350-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.gitbook.io/test-gitbook-open

[argos-ci]

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Build	Status	Details	Updated (UTC)
customers-v2 (Inspect)	👍 Changes approved	10 changed	Jul 14, 2025, 7:57 AM
v2-cloudflare (Inspect)	👍 Changes approved	7 changed	Jul 14, 2025, 8:02 AM
v2-vercel (Inspect)	👍 Changes approved	1 changed	Jul 14, 2025, 8:00 AM