Remove use of Avro `Schema.Parser().setValidate()` to allow use of Avro core 1.12.1 (2.x)

[cowtowncoder]

To allow manual Avro lib override in Jackson 2.x -- to effectively achieve #514 -- we can remove use of method setValidate(): this will allow users to upgrade to Avro lib 1.12.1.
This is something we can do even tho we cannot yet force upgrade as 1.12.1 requires JDK 11 and Jackson 2.x format modules are built on JDK 8
(we may reconsider this limitation for v2.22 but for now it exists).

So: user can override version dependency with:

    <dependency>
      <groupId>org.apache.avro</groupId>
      <artifactId>avro</artifactId>
      <version>1.12.1</version>
    </dependency>

once this is include in 2.21.1 or later (2.22+)

EDIT: Backported in 2.18 (for 2.18.6)