Maintain v1.5 branch for pre-r-rmarkdown builds

[mfansler]

Problem Statement

Starting with v1.6.0, this package added an explicit dependency on r-rmarkdown. That in turn has a pandoc dependency. Unfortunately, pandoc lacks linux-ppc64le builds (see conda-forge/pandoc-feedstock#133). Since this package has over 70 downstream depending packages, all this means that none of those packages can build/install on linux-ppc64le in the R 4.3 context (which only has v1.6 builds). This feedstock thus appears to be in a lynchpin position where issuing a v1.5.4 build for R 4.3 would circumvent this problem.

Proposal

This issue proposes that we maintain a v1.5 branch that continues to build v1.5 for R 4.3. We could leave this open for rebuilding in future R versions, should this situation persist. Should a pandoc become available for linux-ppc64le, then the branch could be removed.

[mfansler]

I plan to extend this to R 4.4.