Build: git backend shouldn't rely on verbose_name to identify latest/stable

[stsewd]

What's the problem this feature will solve?

Around our code base, we always use the slug and machine attribute to identify latest/stable versions, we should use that everywhere, here we are using the verbose name instead.

readthedocs.org/readthedocs/vcs_support/backends/git.py

Lines 135 to 145 in 10d9ef9

	if self.version_machine and self.verbose_name == STABLE_VERBOSE_NAME:
	if self.version_identifier:
	return f"{self.version_identifier}"
	log.error("'stable' version without a commit hash.")
	return None
	tag_name = self.verbose_name
	# For a machine created "latest" tag, the name of the tag is set
	# in the `Version.identifier` field, note that it isn't a commit
	# hash, but the name of the tag.
	if self.version_machine and self.verbose_name == LATEST_VERBOSE_NAME:

Describe the solution you'd like

Use the slug, or maybe refactor this so it doesn't depend several values.

Alternative solutions

None

Additional context

ref #11941

[humitos]

I'm adding this to a future sprint on February because it's related to the work we are doing on changing version's slugs and it seems important to not loose it while we are working/testing that feature.