Updates conjur-authn-k8s version to v0.19.1

[diverdane]

What does this PR do?

This change updates the version of cyberark/conjur-authn-k8s-client that
is used by the Secretless Broker to v0.19.1. In this release of
cyberark/conjur-authn-k8s-client, the Authenticate and
ParseAuthenticationResponse methods were combined into a single method,
so calls to the conjur-authn-k8s-client are changed accordingly.

This change also removes a -mod=readonly flag from the macOS keychain test.
This flag was causing failures in the macOS GitHub actions test because the
macOS builds were requiring changes to go.sum, and having -mod=readonly
causes this to be an error condition.

This change also includes a run of go mod tidy to update go.mod and go.sum.

What ticket does this PR close?

Resolves #1368

Checklists

Change log

• 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

• This PR does not require updating any documentation, or
• Docs (e.g. READMEs) were updated in this PR, and/or there is a follow-on issue to update docs

(For releases only) Manual tests

• Manually tested Keychain provider
• Manually run Kubernetes-Conjur demo with a local Secretless Broker image build of your branch

[doodlesbykumbi]

LGTM! I was thinking we might need a CHANGELOG entry for it but I doubt it since this is an altogether internal change. Needs a rebase though :(

[izgeri]

it might be good to have a changelog message, just because we try to keep our authn-k8s client versions aligned between k8s components when we do suite or DAP releases

[codeclimate]

Code Climate has analyzed commit bbaf72f 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 53.6% (0.0% change).

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