x/tools/gopls/internal/analysis/modernizer: replace "range strings.Fields" with FieldsSeq

[adonovan]

Analogous to for range strings.Split -> for range strings.SplitSeq (see https://go.dev/cl/647438).

(Split out of #70815.)

[gopherbot]

Change https://go.dev/cl/654535 mentions this issue: gopls/internal/analysis/modernize: strings.Fields -> FieldsSeq