Update support to latest requirements for extensions

[cpfergus1]

Summary

Updated Readme with more explicit shell command for running solidus extension . Removed rails > 0.a, bundler has been patched to prevent the issue with resolving dependencies. Bumped Ruby from 2.5 to 2.6, Ruby 2.5 EOL was April 1st 2021, and 2.6 EOL is slated for March 31st 2022

Checklist

• [x] I have structured the commits for clarity and conciseness.
• [ ] I have added relevant automated tests for this change.

[mergify[bot]]

It looks like this PR is missing a label to determine the type of change it introduces. The maintainer should add one of the following labels:

• bug for bugfixes.
• enhancement for new features and improvements.
• documentation for documentation changes.
• security for security patches.
• removed for feature removals.
• infrastructure for internal changes that should not go in the changelog.

Additionally, the maintainer may also want to add one of the following:

• breaking for breaking changes.
• deprecated for feature deprecations.

Once the correct labels have been set, simply remove the needs changelog label label from this PR so I can merge it.

[elia]

@cpfergus1 I rebased this on top of https://github.com/solidusio/solidus_dev_support/pull/188, since they were touching similar files, once that's merged we can rebase on master and merge it if everything checks.

[stale[bot]]

This pull request has been automatically marked as stale. If this pull request is still relevant, please leave any comment (for example, "bump"), and we'll keep it open. We are sorry that we haven't been able to prioritize reviewing it yet. Your contribution is very much appreciated.

[stale[bot]]

Closing this pull request after a prolonged period of inactivity. If this issue is still present in the latest release, please ask for this pull request to be reopened. Thank you!