Migrate GCE runner pools to Regional MIGs (RMIG) across us-central1 (#2870)

Migrate Linux (x86) and Windows runner instance groups from zonal MIGs
(in us-central1-c) to regional MIGs in us-central1 with ANY distribution
shape to prevent capacity stockouts.

### Problem
Upgrading `default_vm` in single-zone `us-central1-c` caused severe
capacity exhaustion across Linux Docker and Windows runners, draining
the active runner fleet as runners terminated after each job.

### Solution
- Migrate runner pools to Regional Managed Instance Groups (RMIGs)
across `us-central1` using `--target-distribution-shape=ANY`. GCE will
dynamically distribute VMs across `us-central1-a`, `b`, `c`, and `f`
based on real-time hardware capacity.
- Inherit `region: us-central1` from `default_vm` for `bk-docker`,
`bk-windows`, and their testing/trusted variants.
- Retain explicit `zone: us-central1-c` on ARM64 pools
(`c4a-standard-8`).
- In `buildkite/create_instances.py`, set
`target_distribution_shape="ANY"` for regional MIG creation and
automatically check for and delete legacy zonal MIGs with the same name
to prevent naming and resource conflicts during migration.

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Co-authored-by: Salma Samy <16399431+SalmaSamy@users.noreply.github.com>