replace `angular.uppercase` with `String.prototype.toUpperCase()` (master/v0.8.13)

[rthaut]

Description

Note that I also created a PR for the develop branch (v1.0), so please do not consider this to be a duplicate (although the fix is the same).

This PR replaces the use of angular.uppercase with String.prototype.toUpperCase(), as AngularJS has removed their helper functions.

This PR is intentionally targeted for the master branch, despite the instructions in the contribution guidelines. This fix is necessary for production systems currently using the latest stable version of AngularJS. Since v0.8.13 is still listed as the primary version on NPM, and since v1.0 has been in development for quite some time, it seems reasonable that this fix should be released for v0.8.13 (or v0.8.14, as the case may be) right away.

Fixes Related issues

• Fixes #970

Checklist

• [x] I have read and understand the CONTRIBUTIONS.md file
• [x] I have searched for and linked related issues
• [ ] I have created test cases to ensure quick resolution of the PR is easier
• [ ] I am NOT targeting main branch
• [x] I did NOT include the dist folder in my PR

@json-schema-form/angular-schema-form-lead

[Anthropic]

@rthaut you could add it to the support/0.x branch as well that is actually the latest version of 0.8.x and has a lot more added to it than master, so if I do a new release I'd probably want to do it from there.

If you can pull that and test it for me I'd feel safer to know it works, there's no tests working on that branch which is why I haven't done any further releases on the code I didn't really write much of myself.