Fix: Narrow react/react-dom peerDependencies to drop false React 16 claim

[balajis-qb]

Description

Linked issue: #6321

Problem

package.json peerDependencies still claim React 16 support
(^16.9.0 || ^17 || ^18 || ^19), but our positioning dependency, @floating-ui/react,
dropped React 16 support as of its 0.27.0 release — the installed
@floating-ui/react already declares peerDependencies: { react: ">=17.0.0" }. Every
React 16 install of react-datepicker has been getting an npm warn peer-dependency
error since @floating-ui/react was bumped past 0.27.0 in #5268, for a React version
we don't actually support end-to-end.

Why floating-ui dropped React 16

Traced to floating-ui/floating-ui#3103
(purely a peerDependencies/changeset bump — no source changed), which closes
floating-ui/floating-ui#3092
(WONTFIX): on React 16, nested FloatingPortals need two clicks to dismiss instead of
one. React 16 attaches synthetic event listeners to document; React 17+ attaches them
to the root container instead, and floating-ui's useDismiss + FloatingPortal
outside-click logic relies on the React 17+ attachment point.

Does this bug reach react-datepicker?

No. src/with_floating.tsx only imports useFloating/autoUpdate/flip/offset/
arrow from @floating-ui/react — pure positioning. It never imports useDismiss or
FloatingPortal; outside-click handling here is our own ClickOutsideWrapper. So the
one documented React-16 regression upstream can't fire through this codebase's usage.

Fix

Metadata-only — no runtime code changes, since the code path that could have broken was
never used:

• package.json — narrow react/react-dom peerDependencies to ^17 || ^18 || ^19 || ^19.0.0-rc, matching @floating-ui/react's own floor.
• yarn.lock — regenerated so the workspace metadata matches.
• README.md — the "Compatibility" table repeated the same stale "React 16 or newer" claim for current versions; added a line clarifying versions after v9.1.0 require React 17+.

Note for reviewers

Narrowing a peerDependencies range is itself semver-breaking for anyone still on
React 16, even though application code is untouched — npm/pnpm peer resolution can go
from a soft warning to a hard install failure for those installs. Worth calling that out
explicitly in release notes rather than shipping it as an invisible patch (i.e. this
probably wants to land as at least a semver-minor with a changelog note, if not paired
with a major bump).

Contribution checklist

• I have followed the contributing guidelines.
• I have formatted my code with Prettier and checked for linting issues with ESLint for code readability.
• yarn type-check, yarn eslint, and yarn test (1485/1485 tests, 43/43 suites) all pass unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.31%. Comparing base (548a1f3) to head (56d4924).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6324      +/-   ##
==========================================
+ Coverage   99.29%   99.31%   +0.02%     
==========================================
  Files          30       30              
  Lines        3822     3822              
  Branches     1648     1668      +20     
==========================================
+ Hits         3795     3796       +1     
+ Misses         26       25       -1     
  Partials        1        1              

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