Remove deprecated flask before_first_request

[albertyw]

Description of the change

Flask v2.3.0 removes its deprecated before_first_request decorator (see changelog). This updates the flask example to replace the decorator with an equivalent example.

Type of change

• 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)
• Maintenance
• New release

Related issues

• Remove deprecated before_first_request rollbar-flask-example#8

Checklists

Development

• Lint rules pass locally
• The code changed/added as part of this pull request has been covered with tests
• All tests related to the changed code pass in development

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers assigned
• Issue from task tracker has a link to this pull request
• Changes have been reviewed by at least one other engineer

[brianr]

Tested and verified 👍