GH-46989: [CI][R] Use Ubuntu 20.04 instead of OpenSUSE for R 4.1 (#46990)

### Rationale for this change

OpenSUSE 15.5 ships old GCC (7.5) that doesn't have enough C++17 support.

### What changes are included in this PR?

Use Ubuntu 20.04 that ships GCC 9.3  instead of OpenSUSE 15.5.

Ubuntu 20.04 reached EOL but we can use it for now.

We discussed why we need OpenSUSE 15.5 based job at #45718 (comment) . We have the job because https://arrow.apache.org/docs/developers/cpp/building.html said "gcc 7.1 and higher should be sufficient".

We need require GCC 9 or later with #46813.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #46989

Lead-authored-by: Sutou Kouhei <[email protected]>
Co-authored-by: Sutou Kouhei <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: kou

dev/tasks/tasks.yml

@@ -1013,7 +1013,7 @@ tasks:
 {% for r_org, r_image, r_tag in [("rhub", "ubuntu-release", "latest"),
                                  ("rocker", "r-ver", "latest"),
                                  ("rstudio", "r-base", "4.2-focal"),
-                                 ("rstudio", "r-base", "4.1-opensuse155")] %}
+                                 ("rstudio", "r-base", "4.1-focal")] %}
   test-r-{{ r_org }}-{{ r_image }}-{{ r_tag }}:
     ci: azure
     template: r/azure.linux.yml