refactor: replace golang.org/x/exp with stdlib

[wangjingcun]

Why this should be merged

Since Go 1.21, the functions in x/exp used here can already be replaced by functions from the standard library

How this works

How this was tested

Need to be documented?

Need to update RELEASES.md?

[qdm12]

Thanks for the pull request! 🎖️
We currently have the code in subnet-evm based off geth v1.13.14 which contains these imports using the exp packages.
However, newer releases of geth contain the same changes as the proposed changes (only use non-exp imports) so these will be picked up automatically when we upgrade subnet-evm with the last geth release, and we want to keep differences with geth v1.13.14 to a minimum to have a smoother upgrade. I'll thus close this, but we appreciate the effort! 👍