Stop pinning Go to patch versions, use stable in CI (#1137)

<!-- Provide a brief summary of your changes -->

## Motivation and Context
<!-- Why is this change needed? What problem does it solve? -->
We keep bumping `go 1.26.x` across the repo every time a new Go patch
drops
## How Has This Been Tested?
<!-- Have you tested this in a real application? Which scenarios were
tested? -->
- **`go.mod` / `deploy/go.mod`**: `go 1.25.8` → `go 1.26` (minor only —
"this module needs Go 1.26")
- **All CI workflows**: `go-version-file: 'go.mod'` → `go-version:
'stable'` — CI always uses the latest stable Go, no more
  bump PRs
- **govulncheck**: removed hardcoded `go-version-input: '1.26.1'` →
`go-version-input: 'stable'`
## Breaking Changes
<!-- Will users need to update their code or configurations? -->

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [ ] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [ ] My code follows the repository's style guidelines
- [ ] New and existing tests pass locally
- [ ] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
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-->

Signed-off-by: Radoslav Dimitrov <radoslav@stacklok.com>

internal/validators/schema.go

@@ -284,7 +284,7 @@ func ConvertJSONPointerToBracketNotation(jsonPointer string) string {
 		// Check if part is a pure number (array index)
 		if _, err := strconv.Atoi(part); err == nil {
 			// It's a numeric index - use bracket notation
-			result.WriteString(fmt.Sprintf("[%s]", part))
+			fmt.Fprintf(&result, "[%s]", part)
 			// Add dot after bracket if next part exists and is a field name (not a number)
 			if i < len(parts)-1 {
 				nextPart := parts[i+1]