chore(docs): Updated warning icon to use emoji (#30979)

Chrish Dunne · web-flow · commit 7494745d07ee · 2021-04-22T10:41:54.000+02:00
The Gatsby site doesn't support converting the `:warning:` string to a symbol, so I replaced it with the emoji character itself to fix this. This is probably a temporary workaround, as ideally the Gatsby site would parse the `:warning:` string into an emoji.
diff --git a/packages/gatsby-image/README.md b/packages/gatsby-image/README.md
@@ -1,4 +1,4 @@
-## :warning: This package is now deprecated
+## ⚠️ This package is now deprecated
 
 The `gatsby-image` package is now deprecated. The new [Gatsby image plugin](https://www.gatsbyjs.com/plugins/gatsby-plugin-image) has better performance, cool new features and a simpler API. See [the migration guide](https://www.gatsbyjs.com/docs/reference/release-notes/image-migration-guide/) to learn how to upgrade.
 

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		`-## :warning: This package is now deprecated`
	`1`	`+## ⚠️ This package is now deprecated`
`2`	`2`
`3`	`3`	The `gatsby-image` package is now deprecated. The new [Gatsby image plugin](https://www.gatsbyjs.com/plugins/gatsby-plugin-image) has better performance, cool new features and a simpler API. See [the migration guide](https://www.gatsbyjs.com/docs/reference/release-notes/image-migration-guide/) to learn how to upgrade.
`4`	`4`