[Bug]: Playground needs to be updated to use a more recent version of Go (minimum Go 1.21)

[lostluck]

What happened?

Validating the staging deployment of the Beam Playground with 2.59.0, we can't modify Go examples, since the minimum Go SDK version is 1.21.0 While the playground seems to be using 1.20.0, so it should just be a docker image change.

Shouldn't be hard to fix, since I believe the playground deploys from head when we push it.

This issue is also causing the Playground Nightly to fail, so it should be resolved.

https://github.com/apache/beam/actions/workflows/beam_Playground_CI_Nightly.yml

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

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