revert(build): drop 4.6.0 as a primary R version option (#186)

#183 added `4.6.0` so the per-minor pass could run under 4.6. It was unnecessary, and it is a footgun.

**Unnecessary:** `build-all-versions` already loops every installed interpreter and runs `local/build-all.R --sensitive-only` for each non-primary minor. The 4.6 pass happens when the pipeline runs with the slot's normal `R_VERSION=4.5.3`. I proposed #183 without reading that loop closely enough.

**Footgun:** `R_VERSION` selects the *primary* minor, and the primary build lands in the **generic** slot. Selecting `4.6.0` for a slot whose generic binaries are 4.5-built would publish 4.6 binaries there and break every 4.5 client — the mirror image of the bug that started all this.

The gaps are being filled by running the pipeline as it already stands (11928 on amd64/resolute).

Reviewed-on: #186

.crow/build-all-versions.yaml

@@ -35,14 +35,16 @@ variables:
       - 'resolute'
     default: '3.24'
   R_VERSION:
+    # The slot's *primary* R minor: what `local/build-all.R` builds into the
+    # generic slot. The loop below already runs `--sensitive-only` for every
+    # other installed minor, so filling a non-primary minor's gap needs this
+    # left alone, not changed.
+    #
+    # 4.6.0 was briefly offered here (#183) and removed: selecting it for a
+    # slot whose generic binaries are 4.5-built would publish 4.6 binaries
+    # into the generic slot and break every 4.5 client.
     description: 'Primary R version under /opt/R.'
-    # The supported window is latest plus the two previous, which the build
-    # images install as R_VERSION_LATEST/PREV1/PREV2. 4.6.0 was missing here,
-    # so no pipeline could run the per-minor pass for it and its slots kept a
-    # backlog: rlang exists for 4.4 and 4.5 on amd64/resolute but not 4.6,
-    # which is how an R 4.6.1 client ended up loading a 4.5.3 binary.
     options:
-      - 4.6.0
       - 4.5.3
       - 4.4.3
     default: 4.5.3