Refactor bastardkb/scylla (#25459)

* Refactor bastardkb/scylla

- Update keyboard aliases accordingly
- Migrate keymap to JSON and better represent physical layout
- scylla/blackpill
  - Migrate common configuration to keyboard level
  - Remove unnecessary or user-specific configuration
  - Migrate some configuration to data-driven
  - Use short SPDX license headers
  - Remove unnecessary rules.mk file
  - Formatting pass on keyboard.json
- scylla/v1
  - Remove likely unneeded version, per qmk#23814
- scylla/v2
  - Remove splinky_2 revision, per qmk#23814
  - Remove splinky_3, stemcell revisions; converter feature should
    be used here
- Rename scylla/v2/elitec -> scylla/promicro, as a converter can
  be used for any pin-compatible microcontrollers

* Small keymap fix

data/mappings/keyboard_aliases.hjson

@@ -68,6 +68,18 @@
     "bakeneko80": {
         "target": "kkatano/bakeneko80"
     },
+    "bastardkb/scylla/v2/elitec": {
+        "target": "bastardkb/scylla/promicro"
+    },
+    "bastardkb/scylla/v2/splinky_2": {
+        "target": "bastardkb/scylla/promicro"
+    },
+    "bastardkb/scylla/v2/splinky_3": {
+        "target": "bastardkb/scylla/promicro"
+    },
+    "bastardkb/scylla/v2/stemcell": {
+        "target": "bastardkb/scylla/promicro"
+    },
     "bastardkb/tbkmini/v2/elitec": {
         "target": "bastardkb/tbkmini/promicro"
     },