feat: remove support for older protobuf versions

.gitmodules

@@ -7,9 +7,6 @@
 [submodule "third_party/fmt"]
 	path = third_party/fmt
 	url = https://github.com/fmtlib/fmt
-[submodule "third_party/abseil-cpp"]
-	path = third_party/abseil-cpp
-	url = https://github.com/abseil/abseil-cpp.git
 [submodule "third_party/datetime"]
 	path = third_party/datetime
 	url = https://github.com/HowardHinnant/date.git

.licenserc.yaml

@@ -12,7 +12,6 @@ header:
     - '.clang-tidy'
     - '.cmake-format'
     - '.licenserc.yaml'
-    - 'third_party/abseil-cpp'
     - 'third_party/antlr4'
     - 'third_party/datetime'
     - 'third_party/fmt'

CMakeLists.txt

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: Apache-2.0
 
-cmake_minimum_required(VERSION 3.24)
+cmake_minimum_required(VERSION 3.27)
 
 # set the project name
 project(substrait-cpp)
@@ -33,25 +33,12 @@ option(
 include_directories(include)
 include_directories(src)
 
-# TODO: Simplify once we can require cmake 3.27 (where CONFIG is default).
-
-# Due to packaging changes we use the combined protobuf/absl packaging if
-# available otherwise we fallback to the older protobuf method.
 find_package(Protobuf QUIET CONFIG)
 if(${Protobuf_FOUND})
   message(STATUS "Modern protobuf library located.")
-  set(ABSL_INCLUDED_WITH_PROTOBUF ON)
 else()
-  find_package(Protobuf QUIET)
-  if(${Protobuf_FOUND})
-    message(STATUS "Legacy protobuf library located.")
-    include_directories(${Protobuf_INCLUDE_DIRS})
-    set(ABSL_INCLUDED_WITH_PROTOBUF OFF)
-  else()
-    message(STATUS "Fetching external protobuf library.")
-    include(third_party/protobuf.cmake)
-    set(ABSL_INCLUDED_WITH_PROTOBUF ON)
-  endif()
+  message(STATUS "Fetching external protobuf library.")
+  include(third_party/protobuf.cmake)
 endif()
 
 add_subdirectory(third_party)

third_party/CMakeLists.txt

@@ -4,10 +4,6 @@
 set(CMAKE_POLICY_DEFAULT_CMP0077 NEW)
 
 set(ABSL_ENABLE_INSTALL ON)
-if(NOT ${ABSL_INCLUDED_WITH_PROTOBUF})
-  set(ABSL_PROPAGATE_CXX_STD ON)
-  add_subdirectory(abseil-cpp)
-endif()
 
 include(datetime.cmake)