go.mod: bump github.com/studio-b12/gowebdav

For https://github.com/studio-b12/gowebdav/pull/87

Fixes #20295

Change-Id: I8ae6ff6969c84fcd510f0e15e0487fbfe9f7c821
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f