CI, the release binaries and swagger take their Go version from go.mod,
which pinned 1.26.7 and so missed the stdlib fixes in 1.26.8 to 1.26.10.
A toolchain line now sets 1.27.3 for them, and the Docker builder uses
the same version. The go line stays at 1.26.7, so source builds with an
older local Go still work.
golangci-lint 2.13.1 can't read Go 1.27.2+ export data, so CI moves to
2.15.0. Its errcheck flags two test cleanups that dropped the server's
exit error; they now check it.
