Use awaitSingle operator instead of awaitFirst.

Operators `awaitFirstXxx` are going to be deprecated. Also, awaitFirst operator has no value on Mono types.

Closes: #1355

Signed-off-by: George Papadopoulos <[email protected]>

Author: GeorgePap-719

spring-data-r2dbc/src/main/kotlin/org/springframework/data/r2dbc/core/ReactiveSelectOperationExtensions.kt

@@ -17,14 +17,15 @@ package org.springframework.data.r2dbc.core
 
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.reactive.asFlow
-import kotlinx.coroutines.reactive.awaitFirstOrNull
 import kotlinx.coroutines.reactive.awaitSingle
+import kotlinx.coroutines.reactor.awaitSingleOrNull
 
 /**
  * Extensions for [ReactiveSelectOperation].
  *
  * @author Mark Paluch
  * @author Oleg Oshmyan
+ * @author George Papadopoulos
  * @since 1.1
  */
 
@@ -50,7 +51,7 @@ suspend fun <T : Any> ReactiveSelectOperation.TerminatingSelect<T>.awaitOne(): T
  * Nullable Coroutines variant of [ReactiveSelectOperation.TerminatingSelect.one].
  */
 suspend fun <T : Any> ReactiveSelectOperation.TerminatingSelect<T>.awaitOneOrNull(): T? =
-		one().awaitFirstOrNull()
+		one().awaitSingleOrNull()
 
 /**
  * Non-nullable Coroutines variant of [ReactiveSelectOperation.TerminatingSelect.first].
@@ -62,7 +63,7 @@ suspend fun <T : Any> ReactiveSelectOperation.TerminatingSelect<T>.awaitFirst():
  * Nullable Coroutines variant of [ReactiveSelectOperation.TerminatingSelect.first].
  */
 suspend fun <T : Any> ReactiveSelectOperation.TerminatingSelect<T>.awaitFirstOrNull(): T? =
-		first().awaitFirstOrNull()
+		first().awaitSingleOrNull()
 
 /**
  * Coroutines variant of [ReactiveSelectOperation.TerminatingSelect.count].