ONYX-14249: Allow activesupport 6 as a dependency

[semyon-estrin]

Desired Outcome

activesupport gem V6.X.X is a possible dependency of conjur-cli. (before that was limited to Ver<=5.0)

Implemented Changes

Change dependency in conjur-cli.gemspec

The end goal is to support rails 6.X.X gem in other conjur related repositories.
activesupport 6.X.X is a dependency to Rails 6.X.X.

Connected Issue/Story

CyberArk internal issue link: ONYX-14249

Changelog

• The CHANGELOG has been updated, or
• This PR does not include user-facing changes and doesn't require a
  CHANGELOG update

Test coverage

• This PR includes new unit and integration tests to go with the code
  changes, or
• The changes in this PR do not require tests

Documentation

• Docs (e.g. READMEs) were updated in this PR
• A follow-up issue to update official docs has been filed here: insert issue ID
• This PR does not require updating any documentation

Behavior

• This PR changes product behavior and has been reviewed by a PO, or
• These changes are part of a larger initiative that will be reviewed later, or
• No behavior was changed with this PR

Security

• Security architect has reviewed the changes in this PR,
• These changes are part of a larger initiative with a separate security review, or
• There are no security aspects to these changes

[codeclimate]

Code Climate has analyzed commit d7d144d and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 51.2% (0.0% change).

View more on Code Climate.

[sashaCher]

LGTM