Use spring.velocity.charset for template encoding

Previously `spring.velocity.charset` only controlled the output encoding
with the templates being loaded with the default encoding. We now
consistently set the same value for both the input and output encodings.

It is still possible to override it to a different value using
`spring.velocity.properties.input.encoding`

Closes gh-3994

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -250,7 +250,7 @@ content into your application; rather pick only the properties that you need.
 	spring.velocity.allow-session-override=false
 	spring.velocity.cache=true
 	spring.velocity.check-template-location=true
-	spring.velocity.charset=UTF-8
+	spring.velocity.charset=UTF-8 # charset for input and output encoding
 	spring.velocity.content-type=text/html
 	spring.velocity.date-tool-attribute=
 	spring.velocity.enabled=true # enable MVC view resolution