Trac #34268: Reduce R to a dummy package, upgrade rpy2 to 3.4.5

We reduce the unmaintained `r` package (stuck at 3.6.3) so that we
detect a system installation of R (>= 3.5) but do not attempt to install
it ourselves.

When R is found as a system package, `rpy2` will be built; otherwise it
won't.

rpy2 3.5.x only supports R >= 4.0 according to
https://rpy2.github.io/doc/v3.5.x/html/overview.html#requirements

Here we upgrade rpy2 to 3.4.5, the latest that still supports R >= 3.5
according to
https://rpy2.github.io/doc/v3.4.x/html/overview.html#requirements

Support for R < 3.5 was already dropped in the rpy 3.3.x series (our
current version of rpy2 is 3.3.6).

According to https://repology.org/project/r/versions, we will thus drop
support for R on the ancient platforms:
 - `ubuntu-trusty` (R 3.0.2; past [[https://wiki.ubuntu.com/Releases|End
of Standard Support]]; EOL April 2024]),
 - `ubuntu-xenial` (R 3.2.3; past End of Standard Support; EOL April
2026),
 - `ubuntu-bionic` (R 3.4.4; End of Standard Support: April 2023)
 - `debian-stretch` (R 3.3.3; past EOL)
 - `fedora-26` (R 3.4.4; past EOL),
as well as on Cygwin. (See #32074)

URL: https://trac.sagemath.org/34268
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): John Palmieri

build/make/Makefile.in

@@ -677,7 +677,7 @@ $(1)-$(4)-no-deps:
 		SAGE_SPKG_WHEELS=$$($(4))/var/lib/sage/wheels \
 		SAGE_INST_LOCAL=$$($(4)) \
 		sage-logger -p 'SAGE_CHECK=$$(SAGE_CHECK_$(1)) $$(SAGE_ROOT)/build/pkgs/$(1)/spkg-install' '$$(SAGE_LOGS)/$(1)-$(2).log' && \
-	    rm -f "$$($(4))/$(SPKG_INST_RELDIR)/$(1)-*" && \
+	    rm -f "$$($(4))/$(SPKG_INST_RELDIR)/$(1)"-* && \
 	    touch "$$($(4))/$(SPKG_INST_RELDIR)/$(1)-$(2)"; \
 	else ( \
 	    echo; \
@@ -704,7 +704,7 @@ $(1)-$(4)-uninstall:
 		. '$$(SAGE_ROOT)/build/bin/sage-build-env-config' && \
 		. '$$(SAGE_ROOT)/build/bin/sage-build-env' && \
 		'$$(SAGE_ROOT)/build/pkgs/$(1)/spkg-uninstall'
-	-rm -f "$$($(4))/$(SPKG_INST_RELDIR)/$(1)-$(2)"
+	-rm -f "$$($(4))/$(SPKG_INST_RELDIR)/$(1)"-*
 
 $(1)-uninstall: $(1)-$(4)-uninstall