Updates for release v21.08.

[tenthirtyam]

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Summary of Pull Request

Type of Pull Request

• This is a bug fix.
• This is an enhancement or feature.
• This is a code style / formatting update.
• This is a documentation update.
• This is a refactoring update.
• This is a something else.
  Please describe:

Context of the Pull Request*

• Moved vsphere.pkrvars.hcl and rhsm.pkrvars.hcl to the builds directory.
• Isolated variables to simplify updates to common settings with builds/common.pkrvars.hcl.
• Refactored all build definitions to use variables and reduce any hard-coded settings.
• All machine image build definitions use the the recommended firmware based on the guest operating system and the minimum vSphere release supported by the repository. EFI Secure Boot is enabled for Red Hat Enterprise Linux 8, CentoOS Linux/Stream 8, AlmaLinux 8, and Microsoft Windows Server 2019/2016. BIOS is enabled Rocky Linux 8.
• Added the manifest post-provisioner to machine image build definitions.
• Moved public keys to variables.
• Added a user for Ansible to Linux machine images with authorized_keys. Uses builds/ansible.pkrvars.hcl. Password is randomized.
• Updated the Packer block to all builds with required_versions >= "1.7.4" for Packer.
• Added the required_plugins to the Packer block for all builds to use the vsphere plugin version = ">= v1.0.1".
• Updated the Packer block for Windows builds with required_plugins and version = "0.14.0" for the Windows-Update plugin.
• All builds automatically run packer init to initialize required plugins before running a build with build.sh.
• Updated Microsoft Windows autounattend.xml files to use the attached VMware Tools .iso for PVSCSI driver.
• Simplified and reduced the script files used by provisioners.
• Patched the script for VNware PhotoN OS 4 due to an issue performing updates with tdnf.
• Patched the script for Ubuntu Server 20.04 LTS to remove cloud-init and remove all netplan configurations before GuestOS customization.
• Removed support for VMware Photon OS 3 machine image builds.
• Removed support for Red Hat Enterprise Linux 7 machine image builds.
• Removed support for CentOS Linux 7 machine image builds.
• Deprecating CentOS Linux 8. Removing in a future update. The CentOS Project has shifted focus from CentOS Linux to CentOS Stream. CentOS Linux 8 EOL: 2021-12-31.
• Deprecating Microsoft Windows Server 2016. Removing in a future update. Microsoft Windows Server 2016 EOL: 2022-01-22.
• Added MAINTAINERS.md.
• Updated README.md.

Related to Existing Issues

Issue Number: None.

Test and Documentation Coverage

• Tests have been been completed (for bug fixes / features).
• Documentation has been added / updated (for bug fixes / features).

Breaking Changes?

• Yes, there are breaking changes.
• No, there are no breaking changes.

[tenthirtyam]

Merge for release 21.08 completed.

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues. If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.