Bump Go to 1.25.8 to fix stdlib CVEs (#253)

## Summary
- Bumps Go version from 1.25.7 to 1.25.8 in `go.mod`
- Fixes CVE-2026-25679 (High), CVE-2026-27142 (Medium), CVE-2026-27139
(Low) in Go stdlib
- Identified via `grype temporalio/temporal-worker-controller:v1.5.0`

## Test plan
- [ ] CI passes with Go 1.25.8
- [ ] After merge, cherry-pick to `release/v1.5.1` branch and tag
`v1.5.1`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Shivs11

go.mod

@@ -1,6 +1,6 @@
 module github.com/temporalio/temporal-worker-controller
 
-go 1.25.7
+go 1.25.8
 
 require (
 	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc

go.work

@@ -1,4 +1,4 @@
-go 1.25.7
+go 1.25.8
 
 use (
 	.

internal/demo/go.mod

@@ -1,6 +1,6 @@
 module github.com/temporalio/temporal-worker-controller/internal/demo
 
-go 1.25.7
+go 1.25.8
 
 require (
 	github.com/prometheus/client_golang v1.23.0

internal/tests/go.mod

@@ -1,6 +1,6 @@
 module github.com/temporalio/temporal-worker-controller/tests
 
-go 1.25.7
+go 1.25.8
 
 require (
 	github.com/temporalio/temporal-worker-controller v1.0.1