Remove stale Microsoft.DBforPostgreSQL/PostgreSQL/ ARM lease (case-collision fix; supersedes #43887)

[vikeshi26]

Removes .github/arm-leases/postgresql/Microsoft.DBforPostgreSQL/PostgreSQL/lease.yaml which case-collides with sibling PostgreSql/lease.yaml on Windows/macOS filesystems and is no longer referenced by the spec layout.

Context

PR #43785 added a PostgreSql/lease.yaml (lowercase l) to accompany the in-flight postgres folder restructure in https://github.com/Azure/azure-rest-api-specs-pr/pull/28858 (a.k.a. #43777, "Refactor(postgresql): migrate to unified folder structure"), which uses PostgreSql as the service folder name. We did not notice at the time that a sibling PostgreSQL/lease.yaml (uppercase SQL) already existed under the same RP.

Both folders coexist fine on Linux git, but Windows/macOS case-insensitive filesystems can only materialize one of them, producing phantom M …/PostgreSQL/lease.yaml (or …/PostgreSql/lease.yaml) entries in git status for any contributor on those platforms after a clone/pull.

Decision

There is currently no service-named folder under specification/postgresql/resource-manager/Microsoft.DBforPostgreSQL/ (only preview/ and stable/ exist on main), so neither casing is in active use today. The in-flight restructure PR #43777 introduces the service folder as PostgreSql/ (lowercase l), and the lease checker in .github/workflows/src/arm-modeling-review/detect-arm-leases.js uses the exact-cased path — so the PostgreSql/ lease is the one needed for that partner PR to clear ARMModelingReviewRequired. The uppercase PostgreSQL/ lease, by contrast, does not match any spec folder that exists or is about to exist.

Therefore:

• Keep: .github/arm-leases/postgresql/Microsoft.DBforPostgreSQL/PostgreSql/lease.yaml (matches partner restructure casing)
• Remove (this PR): .github/arm-leases/postgresql/Microsoft.DBforPostgreSQL/PostgreSQL/lease.yaml (stale, case-only duplicate)

Relationship to #43887

This supersedes #43887 by @timotheeguerin. That PR removes the lowercase PostgreSql/ variant; doing so would re-trigger ARMModelingReviewRequired on #43777 because the lookup path is case-sensitive and the remaining uppercase PostgreSQL/ lease would not be found at the partner-introduced PostgreSql/ path. Suggest closing #43887 in favor of this PR.

Verification

git ls-tree -r origin/main .github/arm-leases/postgresql/Microsoft.DBforPostgreSQL/
# PostgreSql/lease.yaml      ← kept (matches #43777 spec folder casing)
# PostgreSQL/lease.yaml      ← removed by this PR
# DBforPostgreSQL/lease.yaml ← unrelated, kept
# lease.yaml                 ← unrelated, kept

git ls-tree -d origin/main specification/postgresql/resource-manager/Microsoft.DBforPostgreSQL/
# preview/  stable/   (no service folder today)

cc @timotheeguerin @tejaswiMinnu @mikeharder

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