SharingButtonsPreviewButton: add forwardRef flag to connect to make the button reffable

[jsnajdr]

Fixes #37562, a regression introduced by the react-redux upgrade in #37438.

A connected component is used with a ref, and it needs to forward it to the inner component.

[matticbot]

Test live: https://calypso.live/?branch=fix/sharing-preview-button-ref

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~15 bytes added 📈 [gzipped])

name       parsed_size           gzip_size
marketing        +26 B  (+0.0%)      +15 B  (+0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[blowery]

works for me!