Update dependencies to support GCC 15

Previously, compiling using GCC failed with

  In file included from /usr/include/c++/15.1.1/cassert:46,
                   from <ceres-prefix>/third_party/abseil-cpp/absl/container/internal/container_memory.h:18,
                   from <ceres-prefix>/third_party/abseil-cpp/absl/container/flat_hash_map.h:40,
                   from <ceres-prefix>/third_party/abseil-cpp/absl/flags/reflection.h:29,
                   from <ceres-prefix>/third_party/abseil-cpp/absl/flags/reflection.cc:16:
  <ceres-prefix>/third_party/abseil-cpp/absl/container/internal/container_memory.h: In function ‘void* absl::lts_20240116::container_internal::Allocate(Alloc*, size_t)’:
  <ceres-prefix>/third_party/abseil-cpp/absl/container/internal/container_memory.h:66:27: error: ‘uintptr_t’ does not name a type [-Wtemplate-body]
     66 |   assert(reinterpret_cast<uintptr_t>(p) % Alignment == 0 &&
        |                           ^~~~~~~~~
  <ceres-prefix>/third_party/abseil-cpp/absl/container/internal/container_memory.h:31:1: note: ‘uintptr_t’ is defined in header ‘<cstdint>’; this is probably fixable by adding ‘#include <cstdint>’
     30 | #include "absl/utility/utility.h"
    +++ |+#include <cstdint>
     31 |

Additionally, the previous version of Abseil errouneously depends on
googletest when Ceres is consumed which results in issues with
downstream projects. We therefore update both dependencies to avoid
these issues.

Thanks to @adam-ce for figuring out the working versions of Abseil and
googletest.

Fixes #1140

Change-Id: Ie991da04491d42979d51b891a4565cabbb1c604b

.gitmodules

@@ -1,8 +1,8 @@
 [submodule "third_party/googletest"]
 	path = third_party/googletest
 	url = https://github.com/google/googletest.git
-	branch = v1.14.x
+	branch = v1.16.x
 [submodule "third_party/abseil-cpp"]
 	path = third_party/abseil-cpp
 	url = https://github.com/abseil/abseil-cpp
-	branch = lts_2024_01_16
+	branch = lts_2025_01_27