Support for MySQL 8.1 (#143)

Motivation:
MySQL 8.1 has been released

Modification:
Added integration tests for MySQL 8.1

Result:
Ensures compatibility with MySQL 8.1

.github/workflows/ci-integration-tests.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        mysql-version: [ 5.5, 5.6, 5.7, 8.0 ]
+        mysql-version: [ 5.5, 5.6, 5.7, 8.0, 8.1 ]
     name: Integration test with MySQL ${{ matrix.mysql-version }}
     steps:
       - uses: actions/checkout@v3

README.md

@@ -41,6 +41,8 @@ This project is currently being maintained by [@jchrys](https://github.com/jchry
 ![MySQL 5.6 status](https://img.shields.io/badge/MySQL%205.6-pass-blue)
 ![MySQL 5.7 status](https://img.shields.io/badge/MySQL%205.7-pass-blue)
 ![MySQL 8.0 status](https://img.shields.io/badge/MySQL%208.0-pass-blue)
+![MySQL 8.1 status](https://img.shields.io/badge/MySQL%208.1-pass-blue)
+
 
 In fact, it supports lower versions, in the theory, such as 4.1, 4.0, etc.