Increase submission export filename maxlength

Problem/Motivation

When using tokens as filenames for exporting submissions, the tokens can get quite long and will hit the default 128 character limit of textfields.

Steps to reproduce

1. Create a webform, make a few submissions.
2. Go to Results -> Download and select any format that exposes a File name option.
3. Enter a really specific filename using tokens (e.g. [webform_submission:webform:id]--[webform_submission:source-entity:title]--[webform_submission:source-entity:created]--[webform_submission:serial])

Proposed resolution

- Increase maxlength or just make it unbounded.

Remaining tasks

- Review

User interface changes

- File name for results download is increased/unlimited.

API changes

- None

Data model changes

- None