[Refactor][Build] Remove the end-of-support aws-sdk-go v1 from backend/go.mod

[DoDiODev]

What and why to refactor

backend/go.mod still requires github.com/aws/aws-sdk-go (v1). AWS ended
support for the v1 SDK on
2025-07-31:
no more bug fixes, no more security fixes. v1.55.8 is the last release and
deprecates every package, which is how it surfaced in #9192 (lint red with
11× SA1019).

Keeping an unsupported SDK in the dependency tree means:

• any future CVE in it will not be fixed upstream by AWS,
• every Dependabot gomod run that touches it re-triggers the deprecation
  warnings, which currently have to be suppressed by a lint exclusion,
• security scanners will increasingly flag it.

Describe the solution you'd like

1. Migrate the only consumer, backend/plugins/kiro, to aws-sdk-go-v2
   (separate issue: [Refactor][Kiro] Migrate the kiro plugin from aws-sdk-go v1 to aws-sdk-go-v2 #9195).
2. Remove github.com/aws/aws-sdk-go from backend/go.mod / go.sum
   (go mod tidy).
3. Remove the temporary staticcheck exclusion for SA1019 /
   plugins/kiro/ from backend/.golangci.yaml (added in fix(ci): make Dependabot gomod PRs tested and lint-clean #9197).
4. Optional guard: add a depguard rule (or a simple grep in CI) that rejects
   new imports of github.com/aws/aws-sdk-go/, so v1 does not come back
   through another plugin.

@warren830 — since kiro is currently the sole user of the AWS SDK in
DevLake, looping you in here as well. If you plan further AWS-based plugins,
starting them on v2 would avoid repeating this.

Related issues

• [Refactor][Kiro] Migrate the kiro plugin from aws-sdk-go v1 to aws-sdk-go-v2 #9195 (code migration of the kiro plugin)
• build(deps): bump the go-minor-patch group in /backend with 26 updates #9192 (Dependabot go-minor-patch group)
• fix(ci): make Dependabot gomod PRs tested and lint-clean #9197 (temporary lint exclusion + CI paths fix)

Additional context

q_dev, the previous AWS-based plugin, was removed in #9079, so after the
kiro migration no code in the repository depends on v1 anymore.