minor #22137 [Serializer][XmlEncoder] Added a FORMAT constant (Zarcox)
minor #22137 [Serializer][XmlEncoder] Added a FORMAT constant (Zarcox)

This PR was merged into the 3.3-dev branch.

Discussion
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[Serializer][XmlEncoder] Added a FORMAT constant

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Added a FORMAT constant in the XmlEncoder class to normalize it like other Encoder classes.

Commits
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0a0901b8bc [Serializer] Added a FORMAT constant in the XmlEncoder class

symfony/serializer,symfony/Serializer