Rename default Solidus branch to `main`

[waiting-for-dev]

We'll take the occasion of releasing v4.0 to perform the long-due rename of Solidus's default branch from master to main. See #4585 for more context.

We need to perform the following list of tasks in the given order. After renaming the branch (step 3), we might experience some provisional build errors on extensions.

• [x] Rename master references from our dev tools to main. We have several GitHub action automation that only applies to the default branch. (https://github.com/solidusio/solidus/pull/5042)
• [x] Rename references to master in comments, README, etc. Although GitHub will automatically redirect them, it's a good housekeeping chore. (https://github.com/solidusio/solidus/pull/5042)
• [x] Rename master branch to main, just before merging 1 & 2.
• [x] Update CircleCI orb to use main as the default branch. Otherwise, bundle will fail to fetch the unexistent master branch once it's removed. (https://github.com/solidusio/circleci-orbs-extensions/pull/84)
• [x] Update extensions to fetch Solidus from main for development. There's no need to tackle all of them at once, and we can change them opportunistically. However, let's make sure the following ones (including all the ones in the solidusio organization) are updated upfront:
  • [x] solidus_starter_frontend (https://github.com/solidusio/solidus_starter_frontend/pull/337)
  • [x] solidus_stripe (https://github.com/solidusio/solidus_stripe/pull/296)
  • [x] solidus_paypal_commerce_platform (https://github.com/solidusio/solidus_paypal_commerce_platform/pull/198)
  • [x] solidus_braintree (https://github.com/solidusio/solidus_braintree/pull/139)
  • [x] solidus_dev_support (https://github.com/solidusio/solidus_dev_support/pull/204)
  • [x] solidus_support (https://github.com/solidusio/solidus_support/pull/76)
  • [x] solidus_subscriptions (https://github.com/solidusio/solidus_subscriptions/pull/288)
  • [x] solidus_affirm (https://github.com/solidusio/solidus_affirm/pull/75)
  • [x] solidus_bolt (https://github.com/solidusio/solidus_bolt/pull/152)
  • [x] solidus_i18n (https://github.com/solidusio/solidus_i18n/pull/204)
  • [x] solidus_auth_devise (https://github.com/solidusio/solidus_auth_devise/pull/245)
  • [x] solidus_webhooks (https://github.com/solidusio-contrib/solidus_webhooks/pull/15)
• [x] Rename solidus_frontend default branch to main, as some extensions expect the same branch name as Solidus.
• [x] List main and unlist master on stoplight for our API docs.

[waiting-for-dev]

I updated the issue description with all the related PRs.

[kennyadsl]

@waiting-for-dev all the PRs above have been merged, can we close this issue now?

[waiting-for-dev]

Sure thing, closing!