refactoring: replace `pkg/xgenny.NewEmbedWalker` by `io/fs.Sub` in template generators

[tbruyelle]

pkg/xgenny.NewEmbedWalker is used to remove the files prefix from the filesystem passed as a embed.FS. But since go 1.16 we can use io/fs.Sub for this purpose, which is more standard.

As an exemple, this is done in PR #3381 for the app generator.

Once all generators are updated, pkg/xgenny.NewEmbedWalker and related types can probably be removed.