bug #17430 [Serializer] Ensure that groups are strings (dunglas)
bug #17430 [Serializer] Ensure that groups are strings (dunglas)

This PR was squashed before being merged into the 2.7 branch (closes #17430).

Discussion
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[Serializer] Ensure that groups are strings

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Commits
-------

0a3b877 [Serializer] Ensure that groups are strings

symfony/Serializer,symfony/serializer