Bump docker/setup-buildx-action to v2

[trivikr]

This change will fix the following warnings from Setup Docker buildx

Warning: The `save-state` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/
...
Warning: The `set-output` command is deprecated and will be disabled soon. Please upgrade to using Environment Files. For more information see: https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

Example: https://github.com/bluesky-social/social-app/actions/runs/11635138661/job/32403845477

Refs: docker/setup-buildx-action#168

[trivikr]

Is this good to merge?

[trivikr]

The save-state and set-output deprecation warnings are still appearing in workflow runs
https://github.com/bluesky-social/social-app/actions/workflows/build-and-push-bskyweb-aws.yaml

Bumping docker/setup-buildx-action to v2 will remove these warnings