Doc: warnings: since: remove minor version number for uniformity

New warnings are only released in major versions, it seems.
One way or the other, a .1 minor version can always be dropped.

(cherry picked from commit ebcdf3fa)

docs/users_guide/using-warnings.rst

@@ -925,7 +925,7 @@ of ``-W(no-)*``.
     :reverse: -Wno-missing-deriving-strategies
     :category:
 
-    :since: 8.8.1
+    :since: 8.8
 
     The datatype below derives the ``Eq`` typeclass, but doesn't specify a
     strategy. When :ghc-flag:`-Wmissing-deriving-strategies` is enabled,
@@ -963,7 +963,7 @@ of ``-W(no-)*``.
     :reverse: -fnowarn-missing-export-lists
     :category:
 
-    :since: 8.4.1
+    :since: 8.4
 
     .. index::
        single: missing export lists, warning
@@ -1616,7 +1616,7 @@ of ``-W(no-)*``.
     :reverse: -Wno-unused-record-wildcards
     :category:
 
-    :since: 8.10.1
+    :since: 8.10
 
     .. index::
        single: unused, warning, record wildcards
@@ -1638,7 +1638,7 @@ of ``-W(no-)*``.
     :reverse: -Wno-redundant-record-wildcards
     :category:
 
-    :since: 8.10.1
+    :since: 8.10
 
     .. index::
        single: unused, warning, record wildcards