Mark AWAIT_PROMISE as unstable API

[adamziel]

Description

This is a follow-up to #26389 that makes the AWAIT_PROMISE control unstable for now.

Testing instructions

1. Confirm all the tests pass
2. Confirm you are able to use the post editor and the site editor just like before applying this PR
3. Apply this PR on top of Demo of concurrency control issue  #26387 and confirm the issue from its description is not manifested

[github-actions]

Size Change: +8 B (0%)

Total Size: 1.21 MB

Filename	Size	Change
build/core-data/index.js	14.8 kB	+2 B (0%)
build/data-controls/index.js	827 B	+6 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.14 kB	0 B
build/annotations/index.js	3.77 kB	0 B
build/api-fetch/index.js	3.42 kB	0 B
build/autop/index.js	2.83 kB	0 B
build/blob/index.js	665 B	0 B
build/block-directory/index.js	8.71 kB	0 B
build/block-directory/style-rtl.css	943 B	0 B
build/block-directory/style.css	942 B	0 B
build/block-editor/index.js	133 kB	0 B
build/block-editor/style-rtl.css	11.2 kB	0 B
build/block-editor/style.css	11.1 kB	0 B
build/block-library/editor-rtl.css	8.91 kB	0 B
build/block-library/editor.css	8.91 kB	0 B
build/block-library/index.js	147 kB	0 B
build/block-library/style-rtl.css	8.1 kB	0 B
build/block-library/style.css	8.1 kB	0 B
build/block-library/theme-rtl.css	792 B	0 B
build/block-library/theme.css	793 B	0 B
build/block-serialization-default-parser/index.js	1.87 kB	0 B
build/block-serialization-spec-parser/index.js	3.06 kB	0 B
build/blocks/index.js	48 kB	0 B
build/components/index.js	171 kB	0 B
build/components/style-rtl.css	15.3 kB	0 B
build/components/style.css	15.3 kB	0 B
build/compose/index.js	9.87 kB	0 B
build/data/index.js	8.74 kB	0 B
build/date/index.js	31.8 kB	0 B
build/deprecated/index.js	769 B	0 B
build/dom-ready/index.js	571 B	0 B
build/dom/index.js	4.45 kB	0 B
build/edit-navigation/index.js	11.1 kB	0 B
build/edit-navigation/style-rtl.css	881 B	0 B
build/edit-navigation/style.css	885 B	0 B
build/edit-post/index.js	306 kB	0 B
build/edit-post/style-rtl.css	6.43 kB	0 B
build/edit-post/style.css	6.42 kB	0 B
build/edit-site/index.js	22.6 kB	0 B
build/edit-site/style-rtl.css	3.91 kB	0 B
build/edit-site/style.css	3.91 kB	0 B
build/edit-widgets/index.js	26.3 kB	0 B
build/edit-widgets/style-rtl.css	3.16 kB	0 B
build/edit-widgets/style.css	3.16 kB	0 B
build/editor/editor-styles-rtl.css	480 B	0 B
build/editor/editor-styles.css	482 B	0 B
build/editor/index.js	42.5 kB	0 B
build/editor/style-rtl.css	3.85 kB	0 B
build/editor/style.css	3.85 kB	0 B
build/element/index.js	4.62 kB	0 B
build/escape-html/index.js	735 B	0 B
build/format-library/index.js	6.86 kB	0 B
build/format-library/style-rtl.css	547 B	0 B
build/format-library/style.css	548 B	0 B
build/hooks/index.js	2.16 kB	0 B
build/html-entities/index.js	623 B	0 B
build/i18n/index.js	3.57 kB	0 B
build/is-shallow-equal/index.js	712 B	0 B
build/keyboard-shortcuts/index.js	2.52 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/index.js	3.1 kB	0 B
build/list-reusable-blocks/style-rtl.css	476 B	0 B
build/list-reusable-blocks/style.css	476 B	0 B
build/media-utils/index.js	5.31 kB	0 B
build/notices/index.js	1.77 kB	0 B
build/nux/index.js	3.4 kB	0 B
build/nux/style-rtl.css	671 B	0 B
build/nux/style.css	668 B	0 B
build/plugins/index.js	2.56 kB	0 B
build/primitives/index.js	1.43 kB	0 B
build/priority-queue/index.js	789 B	0 B
build/redux-routine/index.js	2.84 kB	0 B
build/reusable-blocks/index.js	3.05 kB	0 B
build/rich-text/index.js	13.4 kB	0 B
build/server-side-render/index.js	2.77 kB	0 B
build/shortcode/index.js	1.69 kB	0 B
build/token-list/index.js	1.27 kB	0 B
build/url/index.js	4.06 kB	0 B
build/viewport/index.js	1.83 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.22 kB	0 B

_{compressed-size-action}

[draganescu]

LGTM! 🚢