Update to Go 1.16

[szh]

Fixes build failing due to goconvey dependency change

The rest is copied from cyberark/secrets-provider-for-k8s#379:

Desired Outcome

Jenkins builds of Docker images do NOT fail with errors such as the following:

[2021-10-26T21:50:18.036Z] #21 1.547 package embed: unrecognized import path "embed": import path does not begin with hostname
[2021-10-26T21:50:26.134Z] #21 8.793 package io/fs: unrecognized import path "io/fs": import path does not begin with hostname
[2021-10-26T21:50:32.671Z] #21 ERROR: executor failed running [/bin/sh -c go get -u github.com/jstemmer/go-junit-report &&     go get github.com/smartystreets/goconvey]: exit code: 1

Implemented Changes

The version of Go used in our base Docker builder container is upgraded from Go v1.15 to Go v1.16.
This seems to eliminate the errors shown above.

Connected Issue/Story

Definition of Done

• Jenkins builds run with no errors.

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

[doodlesbykumbi]

LGTM! Thanks for addressing this.