Make `Instabug.start` ES5-compatible

Description of the change

Remove const usages.

[ ] Bug fix (non-breaking change that fixes an issue)
[ ] New feature (non-breaking change that adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
Related issues

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Checklists

Development
[ ] Lint rules pass locally
[ ] The code changed/added as part of this pull request has been covered with tests
Code review
[ ] This pull request has a descriptive title and information useful to a reviewer
[ ] Issue from task tracker has a link to this pull request