[drake_cmake_external] Upgrade fmt and spdlog pins

[jwnimmer-tri]

See here for the existing pins:

drake-external-examples/drake_cmake_external/CMakeLists.txt

Lines 50 to 51 in 020867b

	ExternalProject_Add(fmt
	URL https://github.com/fmtlib/fmt/archive/refs/tags/6.1.2.tar.gz

drake-external-examples/drake_cmake_external/CMakeLists.txt

Lines 72 to 74 in 020867b

	ExternalProject_Add(spdlog
	DEPENDS fmt
	URL https://github.com/gabime/spdlog/archive/refs/tags/v1.6.0.tar.gz

The request here is to perform a one-time upgrade to re-pin those to match what Drake is currently using in our bazel module file (e.g., for wheel builds):

• bzl bazel_dep(name = "fmt", version = "11.0.2.bcr.1", ...)
• bazel_dep(name = "spdlog", version = "1.15.0.bcr.4", ...)

Of course we can't pin to the "bcr" specific version (which has bazel-specific patches) but anything 11.0.x and 1.15.x are good enough.

Separately, I'll ponder what our policy should be for regular upgrades, but that will be a different ticket.

[tyler-yankee]

I started looking at this, and noting here for future reference that Drake's current symlink scheme for these shared libraries will likely need updating. spdlog started including their minor version in the SOVERSION field in #2376, which was released in v1.11.0. Our build currently passes for 1.10.x but fails to import the shared library for >=1.11.0, because we symlink based on a regex group including the major version only.