Update example repos to 2.21.0

[huonw]

Now that 2.21.0 is released, we should update all the example- repos to use it, by adjusting their pants.toml to have pants_version = "2.21.0": https://github.com/pantsbuild?q=example&type=all&language=&sort=name

• https://github.com/pantsbuild/example-adhoc
• https://github.com/pantsbuild/example-codegen
• https://github.com/pantsbuild/example-django
• https://github.com/pantsbuild/example-docker
• https://github.com/pantsbuild/example-golang
• https://github.com/pantsbuild/example-javascript
• https://github.com/pantsbuild/example-jvm
• https://github.com/pantsbuild/example-kotlin
• https://github.com/pantsbuild/example-python
• https://github.com/pantsbuild/example-serverless
• https://github.com/pantsbuild/example-visibility

(NB. not https://github.com/pantsbuild/example-plugin)

Bonus points (repos that use pants but aren't examples):

• https://github.com/pantsbuild/scie-pants
• https://github.com/pantsbuild/setup

See script: #20799 (comment)

[sureshjoshi]

Ran my script to knock out a bunch of PRs.

• codegen doesn't work on my machine (maybe something about thrift? Didn't look into it yet)
• I don't have go installed on my machine, so that won't work (but I thought we auto-downloaded go binaries, so that's weird - will look into it some day)
• I don't have docker installed on my daily driver, so can't land that
• JVM should be fine - but man, coursier is BRUTAL when working with Lulu network blocker - I hate it so much (edit: did this one manually)

[AlexTereshenkov]

pantsbuild/example-golang#17 is ready for review
pantsbuild/example-docker#18 is ready for review
pantsbuild/example-codegen#27 is ready for review

[AlexTereshenkov]

All repos are now updated.