Backport of Check for nil change during apply into v1.0

v1.0 backport of #29720: skip refreshing deposed during destroy plan

The destroy plan should not require a configured provider (the complete
configuration is not evaluated, so they cannot be configured).

Deposed instances were being refreshed during the destroy plan, because
this instance type is only ever destroyed and shares the same
implementation between plan and walkPlanDestroy. Skip refreshing during
walkPlanDestroy.

add comment about when we call ConfigureProvider

Merge pull request #29704 from hashicorp/backport/jbardin/proposed-new-null-objs/previously-infinite-midge

Backport of objchange: fix ProposedNew from null objects into v1.0

Updated language around contributing modules with overlapping features in the Publishing Modules section: "We welcome..." (all contributions)

Backport of Check required_version as early as possible into v1.0

Merge pull request #29644 from hashicorp/backport/alisdair/fix-refresh-only-with-orphans/inherently-helped-imp

Backport of core: Fix refresh-only interaction with orphans into v1.0

Merge pull request #29599 from hashicorp/backport/laura-add-mrui-to-sidebar/highly-resolved-blowfish

Backport of Add Machine-Readable UI to sidebar into v1.0

Merge pull request #29587 from hashicorp/backport/jbardin/proposed-new-empty-containers/actually-working-viper

Backport of handle empty containers in ProposedNew NestedTypes into v1.0

v1.0 backport 29559

In order to handle optional attributes, the Variable type needs to keep
track of the type constraint for decoding and conversion, as well as the
concrete type for creating values and type comparison.

Since the Type field is referenced throughout the codebase, and for
future refactoring if the handling of optional attributes changes
significantly, the constraint is now loaded into an entirely new field
called ConstraintType. This prevents types containing
ObjectWithOptionalAttrs from escaping the decode/conversion codepaths
into the rest of the codebase.