feat: avm/res/db-for-postgre-sql/flexible-server (#3094)

## Description

Changed "highAvailability" parameter default value to "zoneredundant" to
comply with WAF resiliency pillar.

## Pipeline Reference

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## Type of Change

<!-- Use the checkboxes [x] on the options that are relevant. -->

- [ ] Update to CI Environment or utilities (Non-module affecting
changes)
- [X] Azure Verified Module updates:
- [ ] Bugfix containing backwards-compatible bug fixes, and I have NOT
bumped the MAJOR or MINOR version in `version.json`:
- [ ] Someone has opened a bug report issue, and I have included "Closes
#{bug_report_issue_number}" in the PR description.
- [X] The bug was found by the module author, and no one has opened an
issue to report it yet.
- [ ] Feature update backwards compatible feature updates, and I have
bumped the MINOR version in `version.json`.
- [ ] Breaking changes and I have bumped the MAJOR version in
`version.json`.
  - [X] Update to documentation

## Checklist

- [X] I'm sure there are no other open Pull Requests for the same
update/change
- [X] I have run `Set-AVMModule` locally to generate the supporting
module files.
- [X] My corresponding pipelines / checks run clean and green without
any errors or warnings

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