Update github.com/bitrise-io/bitrise to version 2.21.0

[godrei]

This PR updates github.com/bitrise-io/bitrise to version 2.21.0, to pull the new selective triggering related bitrise.yml data model changes.

I tested this change, by locally running the WFE and saving valid and invalid triggers.

A valid trigger saved without any validation error:

pipelines:
  pl1:
    triggers:
      push:
      - branch: master

while for invalid triggers WFE throws error:

pipelines:
  pl1:
    triggers:
      - push_branch: master

error: Validation failed: Config validation error: failed to get Bitrise config (bitrise.yml) from base 64 data: Failed to parse bitrise config, error: 'triggers': should be a map with 'push', 'pull_request' and 'tag' keys

[bitrise-devs-bot]

Storybook uploaded to: https://storybook.services.bitrise.dev/projects/wfe/pr-1201/