Refactor bastardkb/skeletyl (#25456)

* Refactor bastardkb/skeletyl

- Update keyboard aliases accordingly
- Migrate keymap to JSON
- skeletyl/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
- skeletyl/v1
  - Remove likely unneeded version, per qmk#23812
- skeletyl/v2
  - Remove splinky_2 revision, per qmk#23812
  - Remove splinky_3, stemcell revisions; converter feature should
    be used here
- Rename skeletyl/v2/elitec -> skeletyl/promicro, as a converter can
  be used for any pin-compatible microcontrollers

* Update keyboards/bastardkb/skeletyl/keymaps/default/keymap.json

Co-authored-by: Ryan <fauxpark@gmail.com>

---------

Co-authored-by: Ryan <fauxpark@gmail.com>

data/mappings/keyboard_aliases.hjson

@@ -80,6 +80,18 @@
     "bastardkb/scylla/v2/stemcell": {
         "target": "bastardkb/scylla/promicro"
     },
+    "bastardkb/skeletyl/v2/elitec": {
+        "target": "bastardkb/skeletyl/promicro"
+    },
+    "bastardkb/skeletyl/v2/splinky_2": {
+        "target": "bastardkb/skeletyl/promicro"
+    },
+    "bastardkb/skeletyl/v2/splinky_3": {
+        "target": "bastardkb/skeletyl/promicro"
+    },
+    "bastardkb/skeletyl/v2/stemcell": {
+        "target": "bastardkb/skeletyl/promicro"
+    },
     "bastardkb/tbkmini/v2/elitec": {
         "target": "bastardkb/tbkmini/promicro"
     },