Require MySQL 8.0, PostgreSQL 12, MSSQL 2012 (#27337)

- MySQL 5.7 support and testing is dropped
- MySQL tests now execute against 8.1, up from 5.7 and 8.0
- PostgreSQL 10 and 11 support ist dropped
- PostgreSQL tests now execute against 16, up from 15
- MSSQL 2008 support is dropped
- MSSQL tests now run against locked 2022 version

Fixes: https://github.com/go-gitea/gitea/issues/25657

Ref: https://endoflife.date/mysql
Ref: https://endoflife.date/postgresql
Ref: https://endoflife.date/mssqlserver

## ⚠️ BREAKING ⚠️

Support for MySQL 5.7, PostgreSQL 10 and 11, and MSSQL 2008 is dropped.
You are encouraged to upgrade to supported versions.

---------

Co-authored-by: techknowlogick <techknowlogick@gitea.com>

tests/e2e/README.md

@@ -7,7 +7,6 @@ They can be run with make commands for the appropriate backends, namely:
 make test-sqlite
 make test-pgsql
 make test-mysql
-make test-mysql8
 make test-mssql
 ```
 
@@ -76,7 +75,7 @@ For SQLite:
 make test-e2e-sqlite#example
 ```
 
-For other databases(replace `mssql` to `mysql`, `mysql8` or `pgsql`):
+For other databases(replace `mssql` to `mysql` or `pgsql`):
 
 ```
 TEST_MSSQL_HOST=localhost:1433 TEST_MSSQL_DBNAME=test TEST_MSSQL_USERNAME=sa TEST_MSSQL_PASSWORD=MwantsaSecurePassword1 make test-e2e-mssql#example
@@ -90,4 +89,4 @@ Although the main goal of e2e is assertion testing, we have added a framework fo
 
 VISUAL_TEST=1 will create screenshots in tests/e2e/test-snapshots. The test will fail the first time this is enabled (until we get visual test image persistence figured out), because it will be testing against an empty screenshot folder. 
 
-ACCEPT_VISUAL=1 will overwrite the snapshot images with new images.
+ACCEPT_VISUAL=1 will overwrite the snapshot images with new images.