docs: fix wording in browser support description

[hlongc]

What

Fixes #21144

This PR clarifies the browser support documentation by changing the focus from
browser release dates to feature availability, which better matches the official
Baseline definition.

Before:

These are browsers that were released at least 2.5 years ago.

After:

This means it only uses features which have been available across browsers for at least 2.5 years.

Why

The previous wording "browsers that were released at least 2.5 years ago" is
ambiguous and can be misunderstood as:

• Vite only supports old browsers (>= 2.5 years old)
• Newer browsers are not supported

This is the opposite of what Baseline Widely Available means. According to the
official Baseline definition:

"Widely available: ...It's been available across browsers for at least 2½ years"

The new wording:

• ✅ Emphasizes features (what developers care about)
• ✅ Directly matches the official Baseline definition
• ✅ Removes ambiguity about browser age
• ✅ Clarifies we support modern features that are well-established

Alternatives Explored

• Considered "at most 2.5 years ago" but this still focuses on browsers
• The current approach focusing on features is clearer and matches upstream docs

Checklist

• Read the Contributing Guidelines
• Checked that there isn't already a PR solving this (Guide says "at least 2.5 years ago" but means "at most 2.5 years ago" #21144 has no linked PRs)
• Documentation updated (this IS a documentation fix)
• Tests not applicable (documentation change only)

Additional Context

See discussion in #21144 where community members agreed the focus should be on
"features available for at least 2.5 years" rather than browser ages.

[bluwy]

PR template:

What is this PR solving? Write a clear and concise description.

This is just a one-line change. We do not need some AI-generated description.

Also, have you read the issue discussion for what we've come up as the better wording replacement?