Remove conda buildpacks pin of r-irkernel to 1.2

If r-base is pinned, r-irkernel will resolve to a version that is
compatible with it. But if we pin r-irkernel and not r-base, the
opposite will happen and we will end up with an older version of r-base
than is supported by r-irkernel's modern versions.

I find debugging how versions resolve with conda is really tricky, so
unless we have clear principles of what the pin should be and why, I
strongly advocate we don't have it pinned here.

In this case, having r-irkernel pinned to 1.2 caused us to get stuck at
R version 4.1 instead of going to R 4.2 that is now available.

repo2docker/buildpacks/conda/__init__.py

@@ -362,7 +362,7 @@ def get_env_scripts(self):
                 (
                     "${NB_USER}",
                     r"""
-                ${{MAMBA_EXE}} install -p {0} r-base{1} r-irkernel=1.2 r-devtools -y && \
+                ${{MAMBA_EXE}} install -p {0} r-base{1} r-irkernel r-devtools -y && \
                 ${{MAMBA_EXE}} clean --all -f -y && \
                 ${{MAMBA_EXE}} list -p {0}
                 """.format(
@@ -385,7 +385,7 @@ def get_env_scripts(self):
                 ),
                 (
                     "${NB_USER}",
-                    # Install a pinned version of IRKernel and set it up for use!
+                    # Register the jupyter kernel
                     r"""
                  R --quiet -e "IRkernel::installspec(prefix='{0}')"
                  """.format(