* Lowered go.mod to 1.25.0

This restores support for consumers still using Go 1.25. The previous bump to `go 1.26.4` forced all consumers of this library to use Go 1.26 or newer.
The codebase does not use any Go 1.26-specific features, making the `1.26` language requirement unnecessary.

`go1.25.11` contains the same critical security patches as `go1.26.4` (fixing vulnerabilities in `crypto/x509`, `net/textproto`, and `mime`).
Go's toolchain auto-switching (introduced in Go 1.21) will automatically download and use `go1.25.11` for developers working on this module if their local toolchain is older, ensuring they build with the secure version without forcing a major Go upgrade.

* Changed toolchain to 1.26.4

This makes govulncheck work even for users with go1.26.{0-3}