apps: mark ListTiers and GetTier as deprecated (#719)

apps.go

@@ -384,6 +384,9 @@ func (s *AppsServiceOp) ListRegions(ctx context.Context) ([]*AppRegion, *Respons
 }
 
 // ListTiers lists available app tiers.
+//
+// Deprecated: The '/v2/apps/tiers' endpoint has been deprecated as app tiers
+// are no longer tied to instance sizes. The concept of tiers is being retired.
 func (s *AppsServiceOp) ListTiers(ctx context.Context) ([]*AppTier, *Response, error) {
 	path := fmt.Sprintf("%s/tiers", appsBasePath)
 	req, err := s.client.NewRequest(ctx, http.MethodGet, path, nil)
@@ -399,6 +402,9 @@ func (s *AppsServiceOp) ListTiers(ctx context.Context) ([]*AppTier, *Response, e
 }
 
 // GetTier retrieves information about a specific app tier.
+//
+// Deprecated: The '/v2/apps/tiers/{slug}' endpoints have been deprecated as app
+// tiers are no longer tied to instance sizes. The concept of tiers is being retired.
 func (s *AppsServiceOp) GetTier(ctx context.Context, slug string) (*AppTier, *Response, error) {
 	path := fmt.Sprintf("%s/tiers/%s", appsBasePath, slug)
 	req, err := s.client.NewRequest(ctx, http.MethodGet, path, nil)