Remove references to postgres and use mysql consistently for guide

closes #1161

guides/micronaut-flyway/metadata.json

@@ -8,7 +8,7 @@
   "apps": [
     {
       "name": "default",
-      "features": ["graalvm","data-jdbc","flyway","postgres", "management"]
+      "features": ["graalvm","data-jdbc","flyway","mysql", "management"]
     }
   ]
 }

guides/micronaut-flyway/micronaut-flyway.adoc

@@ -15,15 +15,15 @@ common:cli-or-launch.adoc[]
 [source,bash]
 ----
 mn create-app example.micronaut.micronautguide \
-   --features=data-jdbc,postgres,flyway \
+   --features=data-jdbc,mysql,flyway \
    --build=@build@ --lang=@lang@
 ----
 
 common:build-lang-arguments.adoc[]
 
 common:default-package.adoc[]
 
-NOTE: If you use https://launch.micronaut.io[Micronaut Launch], select "Micronaut Application" as application type and add `postgres`, `data-jdbc`, and `flyway` as features.
+NOTE: If you use https://launch.micronaut.io[Micronaut Launch], select "Micronaut Application" as application type and add `mysql`, `data-jdbc`, and `flyway` as features.
 
 common:annotationprocessors.adoc[]
 
@@ -116,7 +116,7 @@ test:FlywayEndpointTest[]
 callout:micronaut-test[1]
 callout:http-client[2]
 
-common:test-resources-postgres.adoc[]
+common:test-resources-mysql-datasource.adoc[]
 
 common:testApp.adoc[]
 

guides/micronaut-flyway/src/main/resources/application.yml

@@ -5,9 +5,9 @@ micronaut:
 #tag::datasource[]
 datasources:
   default:
-    driverClassName: org.postgresql.Driver  # <1>
-    dialect: POSTGRES # <2>
-    schema-generate: NONE # <3>
+    driverClassName: com.mysql.cj.jdbc.Driver  # <1>
+    db-type: mysql
+    dialect: MYSQL # <2>
 #end::datasource[]
 ---
 #tag::flyway[]

guides/micronaut-flyway/src/main/resources/db/migration/V2__nullable-age.sql

@@ -1 +1 @@
-ALTER TABLE person ALTER COLUMN age DROP NOT NULL;
+ALTER TABLE person MODIFY age int default null;