go.toolchain.branch: switch to Go 1.26

Updates #18682

Change-Id: I1eadfab950e55d004484af880a5d8df6893e85e8
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f