security: upgrade golang to 1.25.6 to fix CVE-2025-61729

[CsatariGergely]

We should update to golang 1.25.6 in all the repos.

• kpt: security: upgrade golang to 1.25.6 to fix CVE-2025-61729 #4358
• krm-functions-sdk: tbd
• krm-functions-catalog: tbd
• .github: N/A No go code in repo
• governance: N/A No go code in repo

[liamfallon]

krm-functions-sdk: kptdev/krm-functions-sdk#748

[liamfallon]

krm-functions-catalog: kptdev/krm-functions-catalog#1214

[liamfallon]

All the PRs are now merged and kpt, krm-functions-sdk, and all the krm functions have been released with the new go version.