ci: remove max-parallel: 3 so one approval covers the whole matrix

[LeonardCareer]

Follow-up to #1266 / #1269 / MSRC #127928 — approval-fatigue fix.

Problem

With max-parallel: 3 on terraform-plan, GitHub creates matrix jobs in waves of 3. Each wave is a separate terraform-privileged deployment request, so approving the environment covers only the current wave. For ~20 tfvars this means 7-8 approval clicks per push (see retest #1267 run #4).

Additionally, the authorize job was a no-op placeholder that only existed to attach environment: terraform-privileged. After #1269 both terraform-test and terraform-plan carry that environment themselves, so authorize no longer serves a purpose.

Fix

Two changes:

1. Delete max-parallel: 3. All matrix jobs now pend simultaneously as one batch. GitHub deduplicates approval per batch, so a single click covers the entire matrix.

2. Delete the authorize job and the needs: authorize references on terraform-test, setup-matrix, and terraform-plan. The approval gate now lives directly on the jobs that actually use the OIDC token (terraform-test, terraform-plan). Same security guarantee — an environment approval is still required before any privileged step can run — just one fewer job in the DAG.

Runner concurrency

telescope has no self-hosted runners; workflows run on the Azure org's shared GitHub-hosted Ubuntu pool. Azure is on GitHub Enterprise Cloud, which caps Ubuntu concurrency at ~500 per org. With ~20 matrix jobs this is well within budget; if org-wide contention ever exhausts the pool, extra jobs simply queue behind the currently-approved batch — they don't fail.

Verification plan

After merge, trigger a Terraform-touching PR and confirm:

• All jobs (terraform-test, setup-matrix, terraform-plan matrix) pend simultaneously
• One click on terraform-privileged approves the whole batch
• No further approval prompts appear as jobs finish

/cc @wonderyl @liyu-ma

[Copilot]

Pull request overview

This PR adjusts the Terraform Integration GitHub Actions workflow to reduce repeated environment approval prompts by allowing the entire terraform-plan matrix to enter the waiting state at once, so a single terraform-privileged approval can cover the full matrix batch.

Changes:

• Remove strategy.max-parallel: 3 from the terraform-plan matrix job so all matrix jobs are created/scheduled simultaneously.
• Preserve existing matrix behavior (fail-fast: false, same matrix source) while changing only the scheduling/approval batching behavior.