Raise the minimum CMake version to 3.15

There are various reasons a later CMake version than 3.8 was
already required, including:

CMakeLists.txt:
  add_link_options(--coverage) # 3.13+

doc/CMakeLists.txt
  include(FetchContent) # 3.11+
  FetchContent_MakeAvailable(FindQtDocs) # 3.14+
  list(PREPEND CMAKE_MODULE_PATH "${findqtdocs_SOURCE_DIR}") # 3.15+

Of course we could work around most of those to still support CMake 3.8,
but 3.15 is already 5 years old, so a pretty reasonable requirement.

CMakeLists.txt

@@ -1,7 +1,7 @@
 # SPDX-FileCopyrightText: 2022-2024 Paul Colby <[email protected]>
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
-cmake_minimum_required(VERSION 3.8...3.29 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.15...3.29 FATAL_ERROR)
 # \todo Replace all of the add_definitions calls with add_compile_definitions
 # if/when we ever set the minimum CMake version to 3.12+.
 message(STATUS "CMake version is ${CMAKE_VERSION}")