Composer: update minimum required PHPUnit Polyfills

[jrfnl]

What?

For compatibility with the WP Core tests, the same (minimum) version of the PHPUnit Polyfills should be required.

This has been v1.1.0 since over a year ago.

Ref: https://core.trac.wordpress.org/ticket/59150

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: jrfnl <jrf@git.wordpress.org>
Co-authored-by: anton-vlasenko <antonvlasenko@git.wordpress.org>
Co-authored-by: desrosj <desrosj@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[anton-vlasenko]

This change looks good to me and seems to match what's already in WordPress Core.