Expand CI matrix to APISIX 3.2/3.9/3.15 and add macOS

[shreemaan-abhishek]

Add an APISIX version matrix to .github/workflows/e2e.yml (3.2 LTS, 3.9, 3.15). Add an OS matrix covering at least linux + macOS for unit and lint (windows is nice-to-have for the completion script path). Pin golangci-lint-action to an exact version rather than latest.

Part of #33

[shreemaan-abhishek]

UPDATE: expanding the matrix is not needed, just setting the version to 3.17.0 is enough