build: Set release version to v1.8.0-beta.0

[travisn]

Description of your changes:
For the new 1.8 branch we create the new v1.8.0-beta.0 manifests and doc updates.

Checklist:

• Commit Message Formatting: Commit titles and messages follow guidelines in the developer guide.
• Skip Tests for Docs: Add the flag for skipping the build if this is only a documentation change. See here for the flag.
• Skip Unrelated Tests: Add a flag to run tests for a specific storage provider. See test options.
• Reviewed the developer guide on Submitting a Pull Request
• Documentation has been updated, if necessary.
• Unit tests have been added, if necessary.
• Integration tests have been added, if necessary.
• Pending release notes updated with breaking and/or notable changes, if necessary.
• Upgrade from previous release is tested and upgrade user guide is updated, if necessary.
• Code generation (make codegen) has been run to update object specifications, if necessary.

[mergify]

Hi @travisn, this pull request was opened against a release branch, is it expected? Normally patches should go in the master branch first and then be backported to release branches.