add 1.25, drop 1.18 support; update deps

[alexandear]

The commit 5a25ad6 states that support for Go 1.18 is dropped.

This PRs goes further and update dependencies and testing configurations.

Changes:

• Update the Go module to require Go version 1.21.0 (was 1.18) in go.mod.
• Bumpe golang.org/x/exp and golang.org/x/sync dependencies.
• Add Go 1.25.0 to the list of tested versions in test_all_versions.sh.
• Update GH actions versions
• Use strategy.matrix to avoid duplicating workflow files.