Update Vinyl Cache to 9.1.0

[github-actions]

Vinyl Cache 9.1.0 is available (current default: 9.0.3).

This PR updates:

• DOWNLOAD_URL in recipe.py and the sample URL in buildout.cfg
• the package version to 9.1.0.0.dev0 (positions 1-3 track the Vinyl Cache release)
• the varnishd -V doctest check to 9.1.x
• CHANGES.rst

New release line (9.0.x → 9.1.x): this PR is a draft and needs manual work before merging.

• Read the upstream changes between 9.0.3 and 9.1.0 (VCL syntax, varnishd options, renamed binaries or tarball)
• Update the 9.0.x mentions in README.rst, recipe.py comments and the doctests
• Check that VMODS_DOWNLOAD_URL (varnish-modules) supports the new line
• Rewrite the changelog entry to describe the change in support

Opened automatically by the vinyl-update workflow.