Pin Bundler at v2.3.26

[john-odonnell]

Desired Outcome

Fix the project's Jenkins build. Related logs:

Step 6/8 : RUN mkdir -p "$TEST_PATH" &&     gem install bundler &&     bundle config --global silence_root_warning 1
 ---> Running in c0d621a3d929
ERROR:  Error installing bundler:
    The last version of bundler (>= 0) to support your Ruby & RubyGems was 2.3.26. Try installing it with `gem install bundler -v 2.3.26`
    bundler requires Ruby version >= 2.6.0. The current ruby version is 2.5.0.

Implemented Changes

The Jenkins build uses a cloudfoundry/cflinuxfs3 container to test the buildpack, where Ruby 2.5 is the default, and highest version available with apt-get. I've pinned Bundler to the version specified in the error message.

PR #160 includes a Ruby 3 update, but this build fails. My PR serves to fix the build until we can devote some time to fixing the Ruby 3 update.

Connected Issue/Story

N/A

Definition of Done

At least 1 todo must be completed in the sections below for the PR to be merged.

Changelog

• [ ] The CHANGELOG has been updated, or
• [x] 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
• [x] 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]
• [x] 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
• [x] 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
• [x] There are no security aspects to these changes