GH-48921: [C++] Bump mimalloc to 3.2.7 (#48826)

### Rationale for this change

Bump mimalloc to the latest version.

On Linux platforms, this seems to make some Parquet reading benchmarks significantly faster.

### Are these changes tested?

By existing CI builds.

### Are there any user-facing changes?

Hopefully no regressions, but memory allocation behavior may change.

* GitHub Issue: #48921

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>

Author: pitrou

cpp/thirdparty/versions.txt

@@ -82,8 +82,8 @@ ARROW_JEMALLOC_BUILD_VERSION=5.3.0
 ARROW_JEMALLOC_BUILD_SHA256_CHECKSUM=2db82d1e7119df3e71b7640219b6dfe84789bc0537983c3b7ac4f7189aecfeaa
 ARROW_LZ4_BUILD_VERSION=v1.10.0
 ARROW_LZ4_BUILD_SHA256_CHECKSUM=537512904744b35e232912055ccf8ec66d768639ff3abe5788d90d792ec5f48b
-ARROW_MIMALLOC_BUILD_VERSION=v3.1.5
-ARROW_MIMALLOC_BUILD_SHA256_CHECKSUM=1c6949032069d5ebea438ec5cedd602d06f40a92ddf0f0d9dcff0993e5f6635c
+ARROW_MIMALLOC_BUILD_VERSION=v3.2.7
+ARROW_MIMALLOC_BUILD_SHA256_CHECKSUM=33621bea902711639b8f83128e64685ffc8224a65443625530747603a2d8726d
 ARROW_NLOHMANN_JSON_BUILD_VERSION=v3.12.0
 ARROW_NLOHMANN_JSON_BUILD_SHA256_CHECKSUM=4b92eb0c06d10683f7447ce9406cb97cd4b453be18d7279320f7b2f025c10187
 ARROW_OPENTELEMETRY_BUILD_VERSION=v1.21.0