Improve first/last docstrings (#39560)

* improve first/last docstrings

improve `first`/`last` docstrings to indicate that two new methods have been added since v1.6 (#34868)

* Update abstractarray.jl

remove the trailing whitespaces

(cherry picked from commit bc2b854)

base/abstractarray.jl

@@ -394,6 +394,9 @@ end
 Get the first `n` elements of the iterable collection `itr`, or fewer elements if `v` is not
 long enough.
 
+!!! compat "Julia 1.6"
+    This method requires at least Julia 1.6.
+
 # Examples
 ```jldoctest
 julia> first(["foo", "bar", "qux"], 2)
@@ -439,6 +442,9 @@ last(a) = a[end]
 Get the last `n` elements of the iterable collection `itr`, or fewer elements if `v` is not
 long enough.
 
+!!! compat "Julia 1.6"
+    This method requires at least Julia 1.6.
+
 # Examples
 ```jldoctest
 julia> last(["foo", "bar", "qux"], 2)