chore(ci): drop Alpine 3.17 and 3.20 build workflows (#667)

## Summary

Trim two Alpine variants from the openemr-devops CI matrix:

**Alpine 3.20 (active, broken):** Recent ccdaservice churn in upstream
openemr master has effectively dropped Node 20 (NODE_MODULE_VERSION 115)
support. `libxmljs2` source build no longer succeeds on Alpine 3.20's
musl/libxml2, so `serveccda.js` silently fails to start and
`CcdaServiceDocumentRequestorTest::testSocket_get` fails with
`socket_connect: Connection refused` (no listener on `:6661`). 3.22,
3.23, and edge use prebuilts and are unaffected.

The runtime-`npm install` model (`docker/openemr/flex/Dockerfile:172`)
means the openemr/openemr CI fixes (openemr/openemr#11620,
openemr/openemr#11728, openemr/openemr#11862 — runner/cache Node-version
alignment) don't transplant here. Drop the matrix slice instead.

**Alpine 3.17 (obsolete, unused):** `flex-3.17` lives under
`docker/openemr/obsolete/` and has no test workflow. The only remaining
lever for it was `build-317.yml`, gated behind `workflow_dispatch` on
master. Other obsolete `flex-*` variants don't carry such workflows —
bring 3.17 in line.

Existing `openemr/openemr:flex-3.17` and `openemr/openemr:flex-3.20-*`
images on Docker Hub are unaffected; this only stops future builds.

## Test plan

- [ ] CI no longer runs `OpenEMR Flex 3.20 Docker Test` or `Flex 3.20
Dockers Build`
- [ ] Workflow dispatch list no longer offers `Flex 3.17 Docker Build`
- [ ] Other matrix entries (3.22, 3.23, edge) continue to pass

Author: kojiromike