boost: add v1.84.0 (#41674)

* Boost: add version 1.84.0
* Conflict with 98/03
* Set C++11 as default
   Starting with 1.84.0, the minimum required is c++11. It has been a very
   long time since 98/03 has been required. It's time to bump the minimum.

var/spack/repos/builtin/packages/boost/package.py

@@ -30,6 +30,7 @@ class Boost(Package):
 
     version("develop", branch="develop", submodules=True)
     version("1.83.0", sha256="6478edfe2f3305127cffe8caf73ea0176c53769f4bf1585be237eb30798c3b8e")
+    version("1.84.0", sha256="cc4b893acf645c9d4b698e9a0f08ca8846aa5d6c68275c14c3e7949c24109454")
     version("1.82.0", sha256="a6e1ab9b0860e6a2881dd7b21fe9f737a095e5f33a3a874afc6a345228597ee6")
     version("1.81.0", sha256="71feeed900fbccca04a3b4f2f84a7c217186f28a940ed8b7ed4725986baf99fa")
     version("1.80.0", sha256="1e19565d82e43bc59209a168f5ac899d3ba471d55c7610c677d4ccf2c9c500c0")
@@ -171,7 +172,7 @@ def libs(self):
 
     variant(
         "cxxstd",
-        default="98",
+        default="11",
         values=(
             "98",
             "11",
@@ -187,6 +188,10 @@ def libs(self):
         multi=False,
         description="Use the specified C++ standard when building.",
     )
+
+    # 1.84.0 dropped support for 98/03
+    conflicts("cxxstd=98", when="@1.84.0:")
+
     variant("debug", default=False, description="Switch to the debug version of Boost")
     variant("shared", default=True, description="Additionally build shared libraries")
     variant(