Warn about non-project mode (#15411)

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# Description

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Please also include relevant motivation and context. -->

In preparation for deprecating non-project mode at some point. This is
phase 1 of encouraging users off of legacy mode.

Any time a user opens a backend in legacy mode (either old states, or
new states with DIY_BACKEND_LEGACY_LAYOUT set) we'll warn that
non-project mode is due to deprecate this year.

They can disable this warning by setting
PULUMI_DIY_BACKEND_ACKNOWLEDGE_LEGACY_WARNING to true.

The next phase (in a few months) will turn this into an error which
they'll be able to opt out of by setting
PULUMI_DIY_BACKEND_ACKNOWLEDGE_LEGACY_ERROR to true.

After that it will be a hard error and all non-project related code will
be removed.

## Checklist

- [x] I have run `make tidy` to update any new dependencies
- [x] I have run `make lint` to verify my code passes the lint check
  - [x] I have formatted my code using `gofumpt`

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- [ ] I have added tests that prove my fix is effective or that my
feature works
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- [x] I have run `make changelog` and committed the
`changelog/pending/<file>` documenting my change
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changelog/pending/20240130--backend-diy--add-a-warning-that-non-project-mode-will-be-deprecated-at-some-point.yaml

@@ -0,0 +1,4 @@
+changes:
+- type: chore
+  scope: backend/diy
+  description: Add a warning that non-project mode will be deprecated at some point.

pkg/backend/diy/backend.go

@@ -302,6 +302,12 @@ func newDIYBackend(
 	var projectMode bool
 	switch meta.Version {
 	case 0:
+		if !env.DIYBackendAcknowledgeLegacyWarning.Value() {
+			msg := "Legacy non-project mode support for DIY backends are due to be deprecated this year." +
+				" Please migrate to project mode by running 'pulumi state upgrade'." +
+				" You can disable this warning by setting PULUMI_DIY_BACKEND_ACKNOWLEDGE_LEGACY_WARNING to true."
+			d.Warningf(diag.RawMessage("", msg))
+		}
 		backend.store = newLegacyReferenceStore(wbucket)
 	case 1:
 		backend.store = newProjectReferenceStore(wbucket, backend.currentProject.Load)

sdk/go/common/env/env.go

@@ -86,6 +86,9 @@ var SkipVersionCheck = env.Bool("AUTOMATION_API_SKIP_VERSION_CHECK",
 
 // Environment variables that affect the DIY backend.
 var (
+	DIYBackendAcknowledgeLegacyWarning = env.Bool("DIY_BACKEND_ACKNOWLEDGE_LEGACY_WARNING",
+		"Disables the warning about legacy non-project mode being due to deprecate.")
+
 	DIYBackendNoLegacyWarning = env.Bool("DIY_BACKEND_NO_LEGACY_WARNING",
 		"Disables the warning about legacy stack files mixed with project-scoped stack files.",
 		env.Alternative("SELF_MANAGED_STATE_NO_LEGACY_WARNING"))