grpc: Remove ErrRetriesExhausted type in favor of error string (#9105)

Addresses: #7023

This change removes the `ErrRetriesExhausted` error type introduced in
#8894. The original requirement was only to add information to the error
message, not to provide the ability to programmatically inspect error
types. This PR keeps the updated error message but removes the new error
type. Since #7023 hasn't been released yet, this is not a breaking
change.

RELEASE NOTES:
* grpc: enhance RPC failure error messages to indicate when retries are
exhausted.

stream.go

diff --git a/stream.go b/stream.go
index 046f549..4aac644 100644
--- a/stream.go
+++ b/stream.go
@@ -148,15 +148,6 @@
 	RecvMsg(m any) error
 }
 
-// ErrRetriesExhausted is returned when an RPC exceeds its configured maximum
-// number of retry attempts.
-//
-// # Experimental
-//
-// Notice: This type is EXPERIMENTAL and may be changed or removed in a
-// later release.
-var ErrRetriesExhausted = errors.New("max retry attempts exhausted")
-
 // NewStream creates a new Stream for the client side. This is typically
 // called by generated code. ctx is used for the lifetime of the stream.
 //
@@ -759,7 +750,7 @@
 		return false, err
 	}
 	if cs.numRetries+1 >= rp.MaxAttempts {
-		return false, fmt.Errorf("stopped after %d attempts: %w: %w", cs.numRetries+1, ErrRetriesExhausted, err)
+		return false, fmt.Errorf("max retries exhausted: failed after %d attempts: %w", cs.numRetries+1, err)
 	}
 
 	var dur time.Duration

test/retry_test.go

diff --git a/test/retry_test.go b/test/retry_test.go
index 62f873b..87c3ec2 100644
--- a/test/retry_test.go
+++ b/test/retry_test.go
@@ -20,7 +20,6 @@
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"io"
 	"net"
@@ -576,8 +575,8 @@
 				t.Fatalf("client: Recv() = %s, %v; want <nil>, error", got, err)
 			} else if status.Code(err) != codes.Unavailable {
 				t.Fatalf("client: Recv() = _, %v; want _, Unavailable", err)
-			} else if !errors.Is(err, grpc.ErrRetriesExhausted) {
-				t.Fatalf("want: ErrRetriesExhausted, got: %v", err)
+			} else if !strings.Contains(err.Error(), "retries exhausted") {
+				t.Fatalf("want: retries exhausted, got: %v", err)
 			}
 
 			serverMu.Lock()
@@ -910,8 +909,8 @@
 			if status.Code(err) != codes.Unavailable {
 				t.Fatalf("client: Recv() = _, %v; want _, Unavailable", err)
 			}
-			if errors.Is(err, grpc.ErrRetriesExhausted) {
-				t.Fatalf("client: Recv() error matches ErrRetriesExhausted, want not match")
+			if strings.Contains(err.Error(), "retries exhausted") {
+				t.Fatalf("client: EmptyCall() failed with an unexpected 'retries exhausted' error: %v", err)
 			}
 
 			// Test unary RPC
@@ -922,8 +921,9 @@
 			if status.Code(err) != codes.Unavailable {
 				t.Fatalf("client: EmptyCall() = _, %v; want _, Unavailable", err)
 			}
-			if errors.Is(err, grpc.ErrRetriesExhausted) {
-				t.Fatalf("client: EmptyCall() error matches ErrRetriesExhausted, want not match")
+
+			if strings.Contains(err.Error(), "retries exhausted") {
+				t.Fatalf("client: EmptyCall() failed with an unexpected 'retries exhausted' error: %v", err)
 			}
 		})
 	}