Upgrade go to 1.26.7 release

Go toolchain upgrade to 1.26.7 on the 5-60-stable builder line.

Backport of !499 (merged) into 5-60-stable, mirroring !500 (merged), which did the same for 5-63-stable.

Note this is a minor bump, 1.25.14 to 1.26.7, not a patch bump. The 5-60 line is still on Go 1.25 while 5-63, 5-65 and 5-68 all moved to 1.26.7 on 2026-09-04.

Why

The Go upgrade guide asks for the same Go version in Omnibus GitLab and Cloud Native GitLab. gitlab-org/build/CNG!3092 (merged) moves the CNG 19-1-stable images to 1.26.7. Without this, a 19.1 patch release would ship package binaries built with 1.25.14 and image binaries built with 1.26.7.

What forced it: 19-1-stable-ee moved GITLAB_ELASTICSEARCH_INDEXER_VERSION to 5.14.14 in gitlab!255991 (merged) (CVE-2026-33816 in jackc/pgx), and v5.14.14 declares go 1.26.0 in its go.mod.

What happens after this merges

It needs tagging as 5.60.5, or the next available patch version. omnibus-gitlab 19-1-stable can then move BUILDER_IMAGE_REVISION from 5.60.3 to that tag, which is the 19.1 equivalent of omnibus-gitlab!9729 (merged) for 19.2.

Verification

Not built locally. The same two values are already in use on 5-63-stable at tag 5.63.2, and shipped in the 19.2.6 and 19.2.7 packages.