Update Go version to 1.25.8 on release-8.5

## Bug Report

**What did you do?**
Checked the current Go toolchain version used on the `release-8.5` branch.

**What did you expect to see?**
The branch should use Go 1.25.8 in the root and scripts modules.

**What did you see instead?**
`release-8.5` is still using Go 1.25.7.

**What version of TiDB Dashboard are you using (`./tidb-dashboard --version`)?**
release-8.5
