Make TextField generator respect max_length

[SergeiJp]

The generator for TextField doesn't respect the max_length set in the model field.
As a result, validating the model through the serializer causes a validation error.

Switch to use gen_string instead of gen_text which respect the max_length and also support MAX_LENGTH constant
if max_length is not set on the field

PR Checklist

• Change is covered with tests
• CHANGELOG.md is updated if needed

[amureki]

Greetings @SergeiJp

Thank you for the contribution, this is very valuable!
I left a small comment on the code; otherwise I think we can proceed with the change. 👍

[amureki]

Great, thank you for the contribution! 🏅

[amureki]

Released in 1.19.4