Unified search placeholders shouldn't use dynamic svg gradients

@PVince81

@PVince81 mentioned in nextcloud/deck#2934 (comment) that svg gradients have quite some performance impact as they are not GPU accelerated.

The unified search UI currently still used them so we should have a look at nextcloud/spreed#4554 (comment) for an alternative implementation.

We no longer have placeholders there, so this is fixed.

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