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GH-42232: [C++] Use non-stale c-ares download URL (#42250)

### Rationale for this change

The URL to download the c-ares dependency is stale. Note: we weren't seeing this bonk on most CI because we have the fallback of our own copy of these. But the R script that downloads all of our thirdparty deps always uses the official URL which is the one that had moved.

Resolves #42232 

### What changes are included in this PR?

Use the appropriate URL, muck with `.` -> `_`

### Are these changes tested?

The current CI should pass + the R maximal offline build should pass too

### Are there any user-facing changes?

More reliable retrieval of all thirdparty dependencies
* GitHub Issue: #42232

Authored-by: Jonathan Keane <jkeane@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>