chore: require Go 1.26

[zeroshade]

Rationale for this change

The next golang.org/x/sync and golang.org/x/sys releases require Go 1.26, blocking dependency updates in #1317 and #1319. Raise the project baseline first so those updates can be rebased without each owning the toolchain migration.

What changes are included in this PR?

• raise the module minimum from Go 1.25 to Go 1.26
• update the default development/container toolchain to Go 1.26.8
• remove Go 1.25 from CI and run linting with Go 1.26
• update the Debian container fallback and Flight SQL driver prerequisite
• fix format diagnostics reported by go test under Go 1.26

Are these changes tested?

• GOTOOLCHAIN=go1.26.8 go test ./...
• GOTOOLCHAIN=go1.26.8 ci/scripts/build.sh $(pwd)
• actionlint -shellcheck= .github/workflows/lint.yml .github/workflows/test.yml
• docker compose config --quiet

Are there any user-facing changes?

Yes. Go 1.26 is now the minimum supported Go version.