fix: Added ext-mbstring to composer.json dependencies

[jasonspriggs]

Changes

Added ext-mbstring to composer.json dependencies. Some systems, such as Heroku, require PHP extension dependencies to be specified in composer.json. This pull request resolves that dependency.

References

https://github.com/auth0/symfony/issues/155#issuecomment-1732696166

Testing

Contributor Checklist

• [x] I agree to adhere to the Auth0 General Contribution Guidelines.
• [x] I agree to uphold the Auth0 Code of Conduct.

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (d10c4a2) 100.00% compared to head (0a687ec) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #730 +/- ## =========================================== Coverage 100.00% 100.00% Complexity 1327 1327 =========================================== Files 62 62 Lines 4613 4613 =========================================== Hits 4613 4613 ``` | [Flag](https://app.codecov.io/gh/auth0/auth0-PHP/pull/730/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | Coverage Δ | | |---|---|---| | [unittestsvalidate](https://app.codecov.io/gh/auth0/auth0-PHP/pull/730/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `100.00% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0#carryforward-flags-in-the-pull-request-comment) to find out more.

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[evansims]

Thanks, @jasonspriggs!