Bump Go toolchain version to 1.25.8

[adombeck]

govulncheck reports the following vulnerabilities in go1.25.7:

Vulnerability #1: GO-2026-4603
    URLs in meta content attribute actions are not escaped in html/template
  More info: https://pkg.go.dev/vuln/GO-2026-4603
  Standard library
    Found in: html/template@go1.25.7
    Fixed in: html/template@go1.25.8
    Example traces found:
Error:       #1: examplebroker/broker.go:1000:14: examplebroker.userInfoFromName calls template.Template.Execute
Error:       #2: pam/integration-tests/ssh_test.go:720:30: integration.startSSHD calls httptest.NewServer, which eventually calls template.Template.ExecuteTemplate

Vulnerability #2: GO-2026-4602
    FileInfo can escape from a Root in os
  More info: https://pkg.go.dev/vuln/GO-2026-4602
  Standard library
    Found in: os@go1.25.7
    Fixed in: os@go1.25.8
    Example traces found:
Error:       #1: internal/users/proc/proc.go:42:33: proc.CheckUserBusy calls os.File.Readdir
Error:       #2: internal/brokers/manager.go:65:29: brokers.NewManager calls os.ReadDir

Vulnerability #3: GO-2026-4601
    Incorrect parsing of IPv6 host literals in net/url
  More info: https://pkg.go.dev/vuln/GO-2026-4601
  Standard library
    Found in: net/url@go1.25.7
    Fixed in: net/url@go1.25.8
    Example traces found:
Error:       #1: internal/testutils/daemon.go:249:29: testutils.StartAuthdWithCancel calls grpc.NewClient, which eventually calls url.Parse
Error:       #2: pam/integration-tests/ssh_test.go:720:30: integration.startSSHD calls httptest.NewServer, which eventually calls url.ParseRequestURI

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.65%. Comparing base (5576f3f) to head (8a649d2).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1309      +/-   ##
==========================================
+ Coverage   80.08%   85.65%   +5.57%     
==========================================
  Files          20      119      +99     
  Lines         984     7669    +6685     
  Branches        0      111     +111     
==========================================
+ Hits          788     6569    +5781     
- Misses        196     1044     +848     
- Partials        0       56      +56     

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[adombeck]

The failure of the Build Debian package job (devel) job is unrelated and tracked here.