Remove support for Ruby 2.5 and 2.6

[szh]

Desired Outcome

According to https://endoflife.date/ruby, Ruby 2.5 and 2.6 are end-of-life. This PR removes CI tests on these version.
Additionally, this PR updates the Github certificate we use in testing because Github has updated their certificate.

Implemented Changes

• Removed CI tests on Ruby version 2.5 and 2.6
• Updated GH cert and fingerprint

Connected Issue/Story

N/A

Definition of Done

• Jenkins passes

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

[rpothier]

LGTM!