Upgrade Postgres from v10 to v15

[john-odonnell]

Desired Outcome

Upgrade Postgres from v10 to v15.

Implemented Changes

• Upgrade postgres.image.tag in values.yaml to '15.4'
• Update Postgres RedHat image to registry.redhat.io/rhel8/postgresql-15
• Fix links to Postgres documentation
• Prepare for release at 2.0.7

Connected Issue/Story

CyberArk internal issue ID: CNJR-2511

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
• 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

[codihuston]

LGTM, thanks so much, John!