fix(security): move the toolchain to go1.26.6

govulncheck fails on main with six standard-library findings, all of them fixed
in go1.26.6: GO-2026-6218, GO-2026-6091, GO-2026-6090, GO-2026-6089,
GO-2026-5972 and GO-2026-5026. None is in a dependency.

The check runs with govulncheck-strict=true, so it is a required check and every
pull request in the repository inherits the failure.

The reusable go-check workflow resolves its toolchain from go-version-file,
which defaults to go.mod, so this line is what CI installs.

Signed-off-by: Sebastian Mendel <info@sebastianmendel.de>

Author: CybotTM

go.mod

@@ -2,7 +2,7 @@ module github.com/netresearch/ofelia
 
 go 1.26
 
-toolchain go1.26.5
+toolchain go1.26.6
 
 require (
 	github.com/armon/circbuf v0.0.0-20190214190532-5111143e8da2