Pin advanced attr() support to the <fallback> syntax

There is no BCD entry for attr() working outside of the content
property for pseudo-elements, but the <fallback> syntax works
because apparently the change was done at the same time.

This matches https://chromestatus.com/feature/4680129030651904

Author: foolip

features/attr.yml

@@ -3,6 +3,8 @@ description: "The `attr()` CSS function, in the context of any property, returns
 spec: https://drafts.csswg.org/css-values-5/#attr-notation
 caniuse: css3-attr
 group: css
+status:
+  compute_from: css.types.attr.fallback
 compat_features:
   # The css.types.attr key makes sense here but is already used in `contents-attr`
   #- css.types.attr

features/attr.yml.dist

@@ -2,9 +2,18 @@
 # Do not edit this file by hand. Edit the source file instead!
 
 status:
-  baseline: false
-  support: {}
+  baseline: low
+  baseline_low_date: 2025-03-31
+  support:
+    chrome: "133"
+    chrome_android: "133"
+    edge: "133"
+    firefox: "119"
+    firefox_android: "119"
+    safari: "18.4"
+    safari_ios: "18.4"
 compat_features:
+  # ⬇️ Same status as overall feature ⬇️
   # baseline: low
   # baseline_low_date: 2025-03-31
   # support:
@@ -24,7 +33,6 @@ compat_features:
   #   edge: "133"
   - css.types.attr.type-or-unit
 
-  # ⬇️ Same status as overall feature ⬇️
   # baseline: false
   # support: {}
   - css.types.attr.type-or-unit.angle