Update docs/_tutorials/migration_v3.md

Co-authored-by: Ankur Oberoi <[email protected]>

.github/workflows/ci-build.yml

@@ -16,7 +16,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [12.x, 14.x]
+        node-version: [12.x, 14.x, 15.x]
 
     steps:
     - uses: actions/checkout@v2

docs/_tutorials/migration_v3.md

@@ -102,7 +102,7 @@ const authorizeFn = async ({ teamId, enterpriseId, isEnterpriseInstall}) => {
 
 ### HTTP Receiver as default
 
-In `@slack/[email protected]`, we have introduced a new default [`HTTPReceiver`](https://github.com/slackapi/bolt-js/issues/670) which replaces the previous default `ExpressReceiver`. This will allow Bolt for JavaScript apps to easily work with other popular web frameworks (Hapi.js, Koa, etc). `ExpressReceiver` is still being shipped with Bolt for JavaScript and their may be some usecases that aren't supported by the new `HTTPReceiver` that `ExpressReceiver` supported. One usecase that isn't supported by `HTTPReceiver` is creating custom routes (ex: create a route to do a health check). For these usecases, we recommend continuing to use `ExpressReceiver` by importing it and create your own instance of it to pass into `App`. See [our documentation on adding custom http routes](https://slack.dev/bolt-js/concepts#custom-routes) for a code sample.
+In `@slack/[email protected]`, we have introduced a new default [`HTTPReceiver`](https://github.com/slackapi/bolt-js/issues/670) which replaces the previous default `ExpressReceiver`. This will allow Bolt for JavaScript apps to easily work with other popular web frameworks (Hapi.js, Koa, etc). `ExpressReceiver` is still being shipped with Bolt for JavaScript and `HTTPReceiver` will not provide all the same functionality. One use case that isn't supported by `HTTPReceiver` is creating custom routes (ex: create a route to do a health check). For these use cases, we recommend continuing to use `ExpressReceiver` by importing the class, and creating your own instance of it, and passing this instance into the constructor of `App`. See [our documentation on adding custom http routes](https://slack.dev/bolt-js/concepts#custom-routes) for an example.
 
 ### @slack/bolt@2.x support schedule