Remove mysql 5.7 from unit tests

- mysql 5.7 is explicitly not supported for [cf-deployment|https://github.com/cloudfoundry/cf-deployment/blob/main/texts/deployment-guide.md#databases]
- fixes #3949 and unblocks #3952

Author: stephanme

.github/workflows/unit_tests.yml

@@ -76,7 +76,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        image: ["mysql:5.7", "mysql:8.0", "mysql:8.2"]
+        image: ["mysql:8.0", "mysql:8.2"]
     services:
       mysql:
         image: ${{ matrix.image }}

.github/workflows/unit_tests_backwards_compatibility.yml

@@ -85,7 +85,7 @@ jobs:
       fail-fast: false
       max-parallel: 3
       matrix:
-        image: ["mysql:5.7", "mysql:8.0", "mysql:8.2"]
+        image: ["mysql:8.0", "mysql:8.2"]
     services:
       mysql:
         image: ${{ matrix.image }}