Support go1.21.0

[chenrui333]

👋 there is some build issue with go4.org/unsafe/assume-no-moving-gc for go1.21.0 build

==> go build -mod=readonly -ldflags=-s -w -X github.com/k3d-io/k3d/v5/version.Version=v5.5.2 -X github.com/k3d-io/k3d/v5/version.K3sVersion=v1.27.
panic: Something in this program imports go4.org/unsafe/assume-no-moving-gc to declare that it assumes a non-moving garbage collector, but your version of go4.org/unsafe/assume-no-moving-gc hasn't been updated to assert that it's safe against the go1.21 runtime. If you want to risk it, run with environment variable ASSUME_NO_MOVING_GC_UNSAFE_RISK_IT_WITH="go1.21" set. Notably, if go1.21 adds a moving garbage collector, this program is unsafe to use.

This commit (https://github.com/go4org/unsafe-assume-no-moving-gc/commit/e7c30c78aeb2bf1ab602f2e789b86db8d9942388) includes the go1.21 build support.

relates to Homebrew/homebrew-core#134468

[iwilltry42]

Should be fixed in v5.6.0 that's just in the pipeline... Thanks for reporting @chenrui333 !