Since we've both concluded the module_variables_optional_attrs experiment
and made experiments available only in alpha releases in the same minor
release, we accidentally made the more general message about experiments
not being available mask the specific message about the experiment being
concluded.

In order to give better feedback to those who were participating in the
experiment in earlier Terraform releases, we'll retain a minimal exception
to our checks to allow the "experiment has concluded" error message to
shine through if and only if that is the only selected experiment.

add deprecation notices to backends

Co-authored-by: Laura Pacilio <83350965+laurapacilio@users.noreply.github.com>

Co-authored-by: Laura Pacilio <83350965+laurapacilio@users.noreply.github.com>

Co-authored-by: Laura Pacilio <83350965+laurapacilio@users.noreply.github.com>

Co-authored-by: Laura Pacilio <83350965+laurapacilio@users.noreply.github.com>

Co-authored-by: Laura Pacilio <83350965+laurapacilio@users.noreply.github.com>

Update commands for new Ubuntu version

Co-authored-by: Robin Norwood <rnorwood@hashicorp.com>

Co-authored-by: Robin Norwood <rnorwood@hashicorp.com>

* Standardise blank lines added to plan for unchanged blocks

* Fix copy-paste error inside NestingList logic

* Fail global required_version check if it contains any prerelease fields

* go mod tidy

* Improve required_version prerelease not supported error string

* Add prerelease version constraint unit tests

* Fix side-effects by populating global diags too soon

remove unmaintained etcd backend to update grpc packages

Added example with function argument expansion

There are no good options for inserting diagnostics into the backend
lookup, or creating a backend which reports it's removal because none of
the init or GetSchema functions return any errors.

Keep a registry of the removed backend so that we can at least notify
users that a backend was removed vs an invalid name.

Update READMEs to help folks figure out how to successfully make docs

Co-authored-by: Kevin Wang <kwangsan@gmail.com>

Update apply page for style, concision, and clarity

Co-authored-by: Nick Fagerlund <nick.fagerlund@gmail.com>

Co-authored-by: Nick Fagerlund <nick.fagerlund@gmail.com>

Co-authored-by: Nick Fagerlund <nick.fagerlund@gmail.com>

Co-authored-by: Nick Fagerlund <nick.fagerlund@gmail.com>

This allows us to remove the manual replace directives
github.com/dgrijalva/jwt-go and google.golang.org/grpc, so that we can
remove the CVE warnings and update the grpc packages.

While the etcdv3 backend is also marked as deprecated, the changes here
are done in a manner to keep that backend working for the time being.