Update go:build comments to go1.21

[thaJeztah]

• relates to https://github.com/moby/moby/pull/47973

Match the minimum version that's specified on our vendor.mod.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 61.73%. Comparing base (860a139) to head (c07cee0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5163 +/- ## ========================================== - Coverage 61.76% 61.73% -0.03% ========================================== Files 297 294 -3 Lines 20768 20763 -5 ========================================== - Hits 12828 12819 -9 - Misses 7024 7026 +2 - Partials 916 918 +2 ```