PHP 7.4 Compatibility: Remove deprecated array/string access syntax

[mdawaffe]

Changes proposed in this Pull Request:

PHP 7.4 deprecates the curly brace array/string access syntax:

$array{1}
$string{3}

https://wiki.php.net/rfc/deprecate_curly_braces_array_access

Switch to use the more-common square bracket syntax:

$array[1]
$string[3]

I used https://gist.github.com/theodorejb/763b83a43522b0fc1755a537663b1863 to find all the curly braces.

Is this a new feature or does it add/remove features to an existing part of Jetpack?

No: Bug fix.

Testing instructions:

?

Proposed changelog entry for your changes:

• Resolve deprecation warnings in anticipation of PHP 7.4.

[matticbot]

Caution: This PR has changes that must be merged to WordPress.com
Hello mdawaffe! These changes need to be synced to WordPress.com - If you 're an a11n, please commandeer, review, and approve D32032-code before merging this PR. Thank you!

[mdawaffe]

I'm not sure how much of this should/can go to various upstream projects.

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: October 1, 2019.
Scheduled code freeze: September 24, 2019

Generated by 🚫 dangerJS against 08b59cf

[kraftbj]

The CSS upstream projects are all generally abandoned or we're on pretty outdated versions (due to older PHP support, even still), so I don't think there is a need to upstream.

[matticbot]

mdawaffe, Your synced wpcom patch D32032-code has been updated.