chore(node): use latest lts

consistent with gitlab/github CI
https://github.com/asdf-vm/asdf-nodejs#resolving-latest-available-lts-version-in-a-script

asdf version in cloudflare pages may not be the latest,
so does not always resolve 'lts' to the latest LTS

asdf currently is unable to install v20 through 'lts'
asdf-vm/asdf-nodejs#369 (comment)

.nvmrc

@@ -1 +1 @@
-20
+lts