Drop support of Go 1.12

Author: shogo82148

.github/workflows/test.yml

@@ -27,8 +27,6 @@ jobs:
               '1.15',
               '1.14',
               '1.13',
-              '1.12',
-              '1.11',
           ]
           mysql = [
               '8.0',

README.md

@@ -40,7 +40,7 @@ A MySQL-Driver for Go's [database/sql](https://golang.org/pkg/database/sql/) pac
   * Optional placeholder interpolation
 
 ## Requirements
-  * Go 1.10 or higher. We aim to support the 3 latest versions of Go.
+  * Go 1.13 or higher. We aim to support the 3 latest versions of Go.
   * MySQL (4.1+), MariaDB, Percona Server, Google CloudSQL or Sphinx (2.2.3+)
 
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