[Form] Added deprecation notice on Form::getErrorsAsString
minor #12654 [Hackday] [Form] Added deprecation notice on Form::getErrorsAsString (stefanosala)

This PR was merged into the 2.7 branch.

Discussion
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[Hackday] [Form] Added deprecation notice on Form::getErrorsAsString

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12611
| License       | MIT
| Doc PR        | -

Commits
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1d45650 [Form] Added deprecation notice on Form::getErrorsAsString