FIX EmailField::validate returns bool

[lekoala]

Description

Just a minor phpdoc update i noticed while working with phpstan

Manual testing steps

extendValidationResult returns bool, so does this

Issues

i'm not sure an issue is required for this :)

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[GuySartorelli]

The CMS 5.4 beta release process has started, so no further releases will be made on the 5 branch. Can you please update this to target the 5.4 branch?