Ruby buildpack now clears previous default buildpack `WEB_CONCURRENCY` values

Change effective on 30 August 2021

Some buildpacks may set a default value for the WEB_CONCURRENCY environment variable during dyno startup.

When multiple buildpacks define default values, the declaration from the last buildpack in the list of multiple buildpacks takes precedence, as this is the primary language of an app.

The Ruby buildpack is now interoperable with other buildpacks in this regard; for example, using heroku/nodejs as the first, and heroku/ruby as the second buildpack, will no longer lead to the Node.js default values for WEB_CONCURRENCY applying at runtime.