Script settings validation breaks mixed version cluster (7.15 + 7.16)

[tvernum]

In #79508 the default value for script.max_compilations_rate was changed from "use-context" to "150/5m".

That means, that validateCacheSetting now treats useContext as false, and rejects any cluster settings on a context.
https://github.com/elastic/elasticsearch/blob/v7.16.0/server/src/main/java/org/elasticsearch/script/ScriptService.java#L349-L360

When in a mixed version cluster, the 7.15 node will accept cluster settings such as

PUT /_cluster/settings
{
  "persistent": {
    "script.context.template.max_compilations_rate": "5000/5m"
  }
}

But the 7.16 version will reject them, and will fail to apply the cluster state on the local node.

Consequently, rolling upgrades with context settings will fail unless script.max_compilations_rate is explicitly set to use-context on the 7.15 node.

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[stu-elastic]

To resolve this, we'll change the interpretation of script.max_compilation_rate.

Users who do not set the script.max_compilation_rate setting and have context settings will implicitly use the context cache. Implicitly using the context cache this way is deprecated and will result in a deprecation warning with the same key as the warning for setting above to "use-context".

We will reject setting script.max_compilation_rate to any value other than "use-context" if the user has context settings.

cc: @colings86.