ci: replace Jenkins with Gitea Actions, modernize DESCRIPTION

- Add real R CMD check workflow via Gitea Actions (rocker/r-ver:4.4)
- Remove Jenkinsfile and demo workflow
- Modernize DESCRIPTION: Authors@R, R >= 4.1.0, URL/BugReports fields,
  move tidycensus from Imports to Suggests, testthat edition 3
- Fix agresti_coull_interval: correct implementation, export it
- Convert get_fips/get_stabbr to use requireNamespace for tidycensus
- Remove civilytics:: self-reference in rnh()

.Rbuildignore

@@ -3,6 +3,5 @@
 ^\.github$
 ^README\.Rmd$
 ^Makefile$
-^Jenkinsfile$
 ^Dockerfile$
 ^LICENSE\.md$