Fix C# syntax errors and netstandard2.0 compatibility in MySql and Postgres drivers. Bump version to 1.9.21

Author: vkuttyp

Directory.Build.props

@@ -7,6 +7,6 @@
     <Authors>vkuttyp</Authors>
     <PackageLicenseExpression>MIT</PackageLicenseExpression>
     <RepositoryUrl>https://github.com/vkuttyp/CosmoSQLClient-Dotnet</RepositoryUrl>
-    <Version>1.9.20</Version>
+    <Version>1.9.21</Version>
   </PropertyGroup>
 </Project>

src/CosmoSQLClient.MySql/MySqlConnection.cs

@@ -358,17 +358,17 @@ public async Task BackupDatabaseAsync(string databaseName, BackupOptions options
         // Logical backup for MySQL (dumping tables to SQL script)
         var sb = new System.Text.StringBuilder();
         sb.AppendLine($"-- MySQL Logical Backup: {databaseName}");
-        sb.AppendLine($"USE \`{databaseName}\`;");
+        sb.AppendLine($"USE `{databaseName}`;");
 
         var tables = await QueryAsync("SHOW TABLES", null, ct).ConfigureAwait(false);
         foreach (var row in tables)
         {
             var tableName = row[0].AsString();
-            var tableData = await QueryTableAsync($"SELECT * FROM \`{tableName}\`").ConfigureAwait(false);
+            var tableData = await QueryTableAsync($"SELECT * FROM `{tableName}`").ConfigureAwait(false);
             sb.AppendLine(SqlDump.GenerateInserts(tableData, tableName));
         }
 
-        await System.IO.File.WriteAllTextAsync(options.DestinationPath, sb.ToString(), ct).ConfigureAwait(false);
+        using (var sw = new System.IO.StreamWriter(options.DestinationPath)) { await sw.WriteAsync(sb.ToString()).ConfigureAwait(false); }
     }
 
     public Task RestoreDatabaseAsync(string databaseName, string sourcePath, CancellationToken ct = default)

src/CosmoSQLClient.Postgres/PostgresConnection.cs

@@ -360,7 +360,7 @@ public async Task BackupDatabaseAsync(string databaseName, BackupOptions options
             sb.AppendLine(SqlDump.GenerateInserts(tableData, tableName));
         }
 
-        await System.IO.File.WriteAllTextAsync(options.DestinationPath, sb.ToString(), ct).ConfigureAwait(false);
+        using (var sw = new System.IO.StreamWriter(options.DestinationPath)) { await sw.WriteAsync(sb.ToString()).ConfigureAwait(false); }
     }
 
     public Task RestoreDatabaseAsync(string databaseName, string sourcePath, CancellationToken ct = default)