Add deprecation note to README and ROADMAP docs

Author: JoelWilcox

README.md

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 [![Maven Central](https://img.shields.io/maven-central/v/com.squareup.anvil/gradle-plugin.svg?label=Maven%20Central)](https://search.maven.org/search?q=g:%22com.squareup.anvil%22)
 [![CI](https://github.com/square/anvil/workflows/CI/badge.svg)](https://github.com/square/anvil/actions?query=branch%3Amain)
 
+### NOTE: Anvil is deprecated and will only be supported for K1 going forward, and development will end when K2 becomes a requirement. See [Anvil Moves to Maintenance Mode](https://github.com/square/anvil/issues/1149) for the full context.
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 > _"When all you have is an anvil, every problem looks like a hammer."_ - [Abraham Maslow](https://en.wikipedia.org/wiki/Law_of_the_instrument)
 
 Anvil is a Kotlin compiler plugin to make dependency injection with [Dagger](https://dagger.dev/)

docs/ROADMAP.md

@@ -1,3 +1,5 @@
+### NOTE: Anvil is deprecated and will only be supported for K1 going forward, and development will end when K2 becomes a requirement. See [Anvil Moves to Maintenance Mode](https://github.com/square/anvil/issues/1149) for the full context. The remainder of this doc is left here for historical purposes.
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 # **Intro**
 
 The last year has been an eventful one for Anvil, with much of the focus on fixing incremental compilation issues and fortifying our integration test capabilities. There has also been a lot of discussion and investigations around the different paths forward in terms of both K2 support and the long-term vision of the library. This doc aims to provide some transparency around where things have settled, where we’re headed, and why various choices were made.