Write helper and validation text for phone extension field

[natalia-fitzgerald]

Task

Write new helper text for the phone extension field to reflect updated requirements.

Context

As a part of this story, we are adjusting the validation requirements for the extension field to accept between 1 and 9 digits.

The following text is included:

• Helper text
• Validation message

[natalia-fitzgerald]

Helper text

Extension must not exceed 9 digits.

Validation text

You must enter a valid extension.

[natalia-fitzgerald]

Updated after dev/design sync

• @angelcardoz