net/http: FileServer no longer serves content for POST [1.20 backport]

[gopherbot]

@rsc requested issue #59375 to be considered for backport to the next 1.20 minor release.

@gopherbot please backport go1.20

[gopherbot]

Change https://go.dev/cl/488635 mentions this issue: [release-branch.go1.20] Revert "net/http: FileServer method check + minimal OPTIONS implementation"

[gopherbot]

Closed by merging 25b4f40 to release-branch.go1.20.