Gallery: Allow gallery to have more columns than total images

[cbirdsong]

Right now I'm trying to lay out a series of galleries with between two and four images, like this:

### Heading

[ image ] [ image ] [ image ]

### Heading

[ image ] [ image ]

### Heading

[ image ] [ image ] [ image ] [ image ]

### Heading

[ image ] [ image ]

I'd like all the images to be the same size since this is listing out candidates for local elections and we don't want to show favoritism by having one candidate's photo appear larger than the other.

However, I can't set the column value to 4 for all the galleries since the gallery block does not let you set the number of columns to a value larger than the number of images:

¹

Currently, it is possible to duplicate a gallery block with four images, remove two of them and end up with markup/block metadata that says there are four columns²:

<!-- wp:gallery {"columns":4,"linkTo":"none"} -->
<figure class="wp-block-gallery has-nested-images columns-4 is-cropped">
<!-- wp:image {"id":56,"aspectRatio":"1","scale":"cover","sizeSlug":"large","linkDestination":"none"} -->
<figure class="wp-block-image size-large"><img src="http://stock.local/wp-content/uploads/2022/01/761eb638-10a7-3784-957b-51c63c28c58c-1024x683.jpg" alt="" class="wp-image-56" style="aspect-ratio:1;object-fit:cover"/></figure>
<!-- /wp:image -->
<!-- /wp:gallery -->

If the input was not restricted then the only change needed to support this would be removing flex-grow: 1; from the stock gallery block CSS.

Footnotes

1. These are not the actual candidates in the election. ↩

2. Made a separate issue about this: Gallery block markup/metadata is not updated when images are removed #55232 ↩