`LowPriorityKeyReads.readableKeyReads` should not exist

Play JSON Version (2.5.x / etc)

3.0.1

API (Scala / Java / Neither / Both)

Scala

Problem

Map serialization/deserialization is broken if the key doesn't have a KeyReads instance.

The problem stems from the LowPriorityKeyReads.readableKeyReads which implies that if you have a Reads[A] then you have KeyReads[A], which does not seem like a logical conclusion. After all, the Reads can expect JsArray, JsObject or any other type, none of which are JsString.

import play.api.libs.json.*

val map = Map((1, 2) -> "foo")

val asJson = Json.prettyPrint(Json.toJson(map)) 
println(asJson)
// Gets serialized to: [ [ [ 1, 2 ], "foo" ] ]

val fromJson = Json.parse(asJson).validate[Map[(Int, Int), String]]
println(fromJson)
// Fails to deserialize, as a failing `KeyReads[(Int, Int)]` will be created from `LowPriorityKeyReads.readableKeyReads`

Suggested solution

Remove LowPriorityKeyReads.readableKeyReads. This breaks backwards compatibility, but I argue that this function is useless (as in it almost never produces a useful Reads instance anyway).

Reproducible Test Case

https://scastie.scala-lang.org/8ePgBH6lQ5i4GmCOPksvyw

Start by locating LowPriorityKeyReads.readableKeyReads in the Play JSON sources and reproduce the Map[(Int, Int), String] case from the linked Scastie example. Check the existing KeyReads and map serialization tests, then verify that the failing key no longer receives an inappropriate fallback and that the reported map case serializes and deserializes successfully.