minor #17539 [Serializer] Update link to HAL specification (jschaedl)

This PR was submitted for the 6.2 branch but it was merged into the 5.4 branch instead.

Discussion
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[Serializer] Update link to HAL specification

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Commits
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1e0800b Update link to HAL specification

Author: javiereguiluz

serializer.rst

@@ -424,4 +424,4 @@ take a look at how this bundle works.
 .. _`OpenAPI`: https://www.openapis.org
 .. _`GraphQL`: https://graphql.org
 .. _`JSON:API`: https://jsonapi.org
-.. _`HAL`: http://stateless.co/hal_specification.html
+.. _`HAL`: https://stateless.group/hal_specification.html