Block Library: Fix React does not recognize isSelected prop in Spacer block

[gziolo]

Description

Fixes #21495.

That error appears in Firefox's JS console when adding the Spacer block.

It doesn't show up in Core 5.4, but does in the plugin as of d85ea0c.

Reproduce

1. Open Firefox and its dev console
2. Browse to the editor, add a Spacer block
3. This will appear in the console:

Warning: React does not recognize the `isSelected` prop on a DOM element. If you intentionally want it to appear in the DOM as a custom attribute, spell it as lowercase `isselected` instead. If you accidentally passed it from a parent component, remove it from the DOM element.
    in div (created by t)
    in t (created by Qi)
    ...

I replaced isSelected with the existing showHandle prop. isSelected was introduced in #20746.

Testing

1. Open Firefox and its dev console
2. Browse to the editor, add a Spacer block
3. There should be no React warning on JS console related to isSelected prop.
4. Ensure that you can still change the height of the Spacer block.

[github-actions]

Size Change: -1 B

Total Size: 816 kB

Filename	Size	Change
build/block-library/editor-rtl.css	7.04 kB	+7 B (0%)
build/block-library/editor.css	7.04 kB	+6 B (0%)
build/block-library/index.js	114 kB	+2 B (0%)
build/components/style-rtl.css	16.9 kB	-8 B (0%)
build/components/style.css	16.9 kB	-8 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.02 kB	0 B
build/annotations/index.js	3.62 kB	0 B
build/api-fetch/index.js	4.08 kB	0 B
build/autop/index.js	2.82 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	6.23 kB	0 B
build/block-directory/style-rtl.css	760 B	0 B
build/block-directory/style.css	761 B	0 B
build/block-editor/index.js	106 kB	0 B
build/block-editor/style-rtl.css	10.2 kB	0 B
build/block-editor/style.css	10.2 kB	0 B
build/block-library/style-rtl.css	7.14 kB	0 B
build/block-library/style.css	7.14 kB	0 B
build/block-library/theme-rtl.css	683 B	0 B
build/block-library/theme.css	685 B	0 B
build/block-serialization-default-parser/index.js	1.88 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	48.1 kB	0 B
build/components/index.js	179 kB	0 B
build/compose/index.js	6.66 kB	0 B
build/core-data/index.js	11.4 kB	0 B
build/data-controls/index.js	1.29 kB	0 B
build/data/index.js	8.42 kB	0 B
build/date/index.js	5.47 kB	0 B
build/deprecated/index.js	772 B	0 B
build/dom-ready/index.js	568 B	0 B
build/dom/index.js	3.1 kB	0 B
build/edit-navigation/index.js	3.54 kB	0 B
build/edit-navigation/style-rtl.css	485 B	0 B
build/edit-navigation/style.css	485 B	0 B
build/edit-post/index.js	27.6 kB	0 B
build/edit-post/style-rtl.css	12.2 kB	0 B
build/edit-post/style.css	12.2 kB	0 B
build/edit-site/index.js	10.9 kB	0 B
build/edit-site/style-rtl.css	5.11 kB	0 B
build/edit-site/style.css	5.11 kB	0 B
build/edit-widgets/index.js	7.49 kB	0 B
build/edit-widgets/style-rtl.css	4.67 kB	0 B
build/edit-widgets/style.css	4.66 kB	0 B
build/editor/editor-styles-rtl.css	428 B	0 B
build/editor/editor-styles.css	431 B	0 B
build/editor/index.js	43.4 kB	0 B
build/editor/style-rtl.css	3.27 kB	0 B
build/editor/style.css	3.27 kB	0 B
build/element/index.js	4.65 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	7.63 kB	0 B
build/format-library/style-rtl.css	502 B	0 B
build/format-library/style.css	502 B	0 B
build/hooks/index.js	2.13 kB	0 B
build/html-entities/index.js	622 B	0 B
build/i18n/index.js	3.56 kB	0 B
build/is-shallow-equal/index.js	710 B	0 B
build/keyboard-shortcuts/index.js	2.51 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/index.js	3.12 kB	0 B
build/list-reusable-blocks/style-rtl.css	226 B	0 B
build/list-reusable-blocks/style.css	226 B	0 B
build/media-utils/index.js	5.29 kB	0 B
build/notices/index.js	1.79 kB	0 B
build/nux/index.js	3.4 kB	0 B
build/nux/style-rtl.css	616 B	0 B
build/nux/style.css	613 B	0 B
build/plugins/index.js	2.67 kB	0 B
build/primitives/index.js	1.5 kB	0 B
build/priority-queue/index.js	789 B	0 B
build/redux-routine/index.js	2.84 kB	0 B
build/rich-text/index.js	14.8 kB	0 B
build/server-side-render/index.js	2.68 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.28 kB	0 B
build/url/index.js	4.02 kB	0 B
build/viewport/index.js	1.84 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.18 kB	0 B

_{compressed-size-action}

[jorgefilipecosta]

The PR worked well on my tests 👍

[SergioEstevao]

Sorry to comment on this late, but I just tested on RN mobile and it's working fine there.