Update mysql version to 8.0 in misk CI

Mysql 5.7 is EOL and upgrading to 8.0 keeps us on a more relevant
version that is better supported with skeema and other tools.

GitOrigin-RevId: ece16d3bbcd041b97652b43e0027b6797f438a52

.github/workflows/Build.yml

@@ -36,7 +36,7 @@ jobs:
 
     services:
       mysql:
-        image: mysql:5.7
+        image: mysql:8.0
         ports:
           - 3306:3306
         env:

CONTRIBUTING.md

@@ -23,7 +23,7 @@ misk-hibernate tests expect a mysql server running on `localhost:3306` with no p
 the root user. You might stand up a server with a docker image, e.g.
 
 ```shell
-docker run -d --rm --name "mysql-57" -p 3306:3306 -e MYSQL_ALLOW_EMPTY_PASSWORD=true -e MYSQL_LOG_CONSOLE=true mysql:5.7 --sql-mode=""
+docker run -d --rm --name "mysql-80" -p 3306:3306 -e MYSQL_ALLOW_EMPTY_PASSWORD=true -e MYSQL_LOG_CONSOLE=true mysql:8.0 --sql-mode=""
 ```
 
 Misk may download these Docker images as part of its tests. Because tests can time out, pre-downloading these can help resolve timeouts.