[Trivial] Run go mod tidy - bumps go version to 1.21

[blmhemu]

Description

go mod tidy bumps the go version and the toolchain. Updated the dockerfile to reflect the same.

Tests

Not required.

[CLAassistant]

All committers have signed the CLA.

[mnvr]

Hey @blmhemu, firstly thank you for the pull request!

For the go version (and for the versions of other dependencies), we try to consciously and intentionally update them manually, along with doing some sort of test run. So while your PR is great, and is the same thing we'd have done, right now we're not thinking of updating the go version.

Apologies, but I'll close this PR for now. Thanks!