.Net: Fix postgres memory store dispose

[JadynWong]

Motivation and Context

Fix #3121

Description

Convert to IDisposable to execute Dispose.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[JadynWong]

Create new classlibary with netstandard2.0 framework.

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>netstandard2.0</TargetFramework>
    <LangVersion>10</LangVersion>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.SemanticKernel" Version="1.0.0-beta1" />
    <PackageReference Include="Microsoft.SemanticKernel.Connectors.Memory.Postgres" Version="1.0.0-beta1" />
  </ItemGroup>
  
</Project>

Copy the PostgresMemoryStore code into classlibary, naming it MyPostgresMemoryStore.

Reproduce the problem

Reslove the problem

note

If use MyPostgresMemoryStore directly in console(.net7), that can't reproduce the problem. I can't pinpoint the exact problem, but it seems to be related to TargetFramework. Maybe it should be solved by npgsql.

Reference npgsql/efcore.pg#2834