Widgets: Revert [34465], as it introduced a regression, making the `$index`...

Widgets: Revert [34465], as it introduced a regression, making the `$index` argument of `dynamic_sidebar()` case-sensitive.

Fixes #34995 for trunk. See #23423.
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git-svn-id: https://core.svn.wordpress.org/trunk@36096 1a063a9b-81f0-0310-95a4-ce76da25c4cd