Document the procedure of upgrading to the new Kotlin version (#4557)

Fixes #4521

Author: dkhalanskyjb

KOTLIN_UPGRADE.md

@@ -0,0 +1,38 @@
+# Checklist for upgrading to the new Kotlin version
+
+1. Create a new branch based off `develop`:
+   ```sh
+   git fetch
+   git checkout -b kotlin-<version> origin/develop
+   ```
+
+1. In `gradle.properties`,
+   set `kotlin_version` to the `<version>`.
+
+2. Likewise, set `kotlin_version` in `integration-testing/gradle.properties`.
+
+3. In `README.md`,
+   replace the occurrences of the old version with the `<version>`.
+   These are:
+   - A badge at the top of the file.
+   - The Kotlin version corresponding to the latest release,
+     mentioned in the first paragraph.
+   - Maven and Gradle configuration blocks.
+
+4. In `README.md`, find the link to https://pl.kotl.in/ and open it.
+   Set the compiler version of Kotlin Playground to the same major version as `<version>`,
+   click "Copy link", and replace the original link in `README.md` with the new one.
+
+5. Perform a full build: `gradle clean check publishToMavenLocal`,
+   followed by `gradle clean check` in the `integration-testing` directory.
+   Fix any issues that arise.
+
+6. Commit and push all changes to GitHub:
+   ```sh
+   git commit -am 'Upgrade to Kotlin <version>'
+   git push -u origin kotlin-<version>
+   ```
+
+7. Wait for the TeamCity continuous integration builds to finish.
+
+8. Open a new pull request, receive approval, and merge.

README.md

@@ -14,14 +14,14 @@ This is a companion version for the Kotlin `2.2.20` release.
 ```kotlin
 suspend fun main() = coroutineScope {
     launch { 
-       delay(1000)
+       delay(1.seconds)
        println("Kotlin Coroutines World!") 
     }
     println("Hello")
 }
 ```
 
-> Play with coroutines online [here](https://pl.kotl.in/Wf3HxgZvx)
+> Play with coroutines online [here](https://pl.kotl.in/xdbDf0NNk)
 
 ## Modules