Update openjdk to version 21

[dnerdy]

Summary:

This PR updates the dotfiles to install openjdk@21 instead of openjdk@11. The
new datastore emulator requires openjdk@21[1]. Our java code (specifically, our
dataflow jobs) have been updated to be compatible with openjdk@21[2].

[1] https://khanacademy.slack.com/archives/CDHPTMSTB/p1764100041132309
[2] https://khanacademy.slack.com/archives/C02NV1DPJ/p1761164767463419?thread_ts=1758663304.942309&cid=C02NV1DPJ

Issue: https://khanacademy.slack.com/archives/C02NV1DPJ/p1764186772736169?thread_ts=1758663304.942309&cid=C02NV1DPJ

Test plan:

Making this change I ran:

java --version

which output:

openjdk 11.0.27 2025-04-15
OpenJDK Runtime Environment Homebrew (build 11.0.27+0)
OpenJDK 64-Bit Server VM Homebrew (build 11.0.27+0, mixed mode)

After making this change I ran the dotfiles:

cd ~/khan/devtools/khan-dotfiles
make

This ran successfully.

I then opened a new terminal window, and ran the following again:

java --version

which output:

openjdk 21.0.9 2025-10-21
OpenJDK Runtime Environment Homebrew (build 21.0.9)
OpenJDK 64-Bit Server VM Homebrew (build 21.0.9, mixed mode, sharing)

I also ran:

CLOUDSDK_CORE_PROJECT=khan-academy gcloud beta emulators datastore start --data-dir=/tmp/datastore --host-port=127.0.0.1:5555

and the datastore emulator started up on port 5555 without issue.

(Note: I needed to update by macOS to version 26.1 to successfully run
khan-dotfiles, but after doing that everything ran just fine.)

[csilvers]

Thank you!