chore: remove the stale ChangeLog.md and docs/json.gif

ChangeLog.md was last generated in January 2025 and still ends at
"unreleased (2024-12-22)"; 3.12.0 shipped without an entry. The release
notes live on the GitHub releases page and in docs/mkdocs/docs/home/releases/.
Remove the file, the Makefile target that called github_changelog_generator,
and the README credit for that tool.

docs/json.gif is an old copy of the feature slideshow that nothing
references; the home page uses the SVGs built by docs/mkdocs/scripts/slideshow/.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

Makefile

@@ -1,4 +1,4 @@
-.PHONY: all amalgamate check-amalgamation check_build_options pretty install_astyle clean ChangeLog.md release update_hedley update_hedley_undef BUILD.bazel natvis macro_builder_check run_benchmarks pvs_studio serve_header reuse spdx
+.PHONY: all amalgamate check-amalgamation check_build_options pretty install_astyle clean release update_hedley update_hedley_undef BUILD.bazel natvis macro_builder_check run_benchmarks pvs_studio serve_header reuse spdx
 
 ##########################################################################
 # configuration
@@ -40,7 +40,6 @@ MACRO_SCOPE_HPP=include/nlohmann/detail/macro_scope.hpp
 all:
 	@echo "amalgamate - amalgamate files single_include/nlohmann/json{,_fwd,_literals,_view}.hpp from the include/nlohmann sources"
 	@echo "BUILD.bazel - regenerate the Bazel BUILD file from the include/nlohmann sources"
-	@echo "ChangeLog.md - generate ChangeLog file"
 	@echo "check-amalgamation - check whether sources have been amalgamated and BUILD.bazel is up to date"
 	@echo "check_build_options - check that the Meson build and the pkg-config files offer the options of the CMake target"
 	@echo "clean - remove built files"
@@ -175,22 +174,6 @@ check-amalgamation:
 BUILD.bazel:
 	cmake -P cmake/scripts/gen_bazel_build_file.cmake
 
-##########################################################################
-# ChangeLog
-##########################################################################
-
-# Create a ChangeLog based on the git log using the GitHub Changelog Generator
-# (<https://github.com/github-changelog-generator/github-changelog-generator>).
-
-# variable to control the diffs between the last released version and the current repository state
-NEXT_VERSION ?= "unreleased"
-
-ChangeLog.md:
-	github_changelog_generator -o ChangeLog.md --user nlohmann --project json --simple-list --release-url https://github.com/nlohmann/json/releases/tag/%s --future-release $(NEXT_VERSION)
-	$(SED) -i 's|https://github.com/nlohmann/json/releases/tag/HEAD|https://github.com/nlohmann/json/tree/HEAD|' ChangeLog.md
-	$(SED) -i '2i All notable changes to this project will be documented in this file. This project adheres to [Semantic Versioning](http://semver.org/).' ChangeLog.md
-
-
 ##########################################################################
 # Release files
 ##########################################################################

README.md

@@ -1909,7 +1909,6 @@ The library itself consists of a single header file licensed under the MIT licen
 - [**cpplint**](https://github.com/cpplint/cpplint) for style checks
 - [**doctest**](https://github.com/doctest/doctest) for the unit tests
 - [**Flawfinder**](https://dwheeler.com/flawfinder/) for static analysis
-- [**GitHub Changelog Generator**](https://github.com/github-changelog-generator/github-changelog-generator) to generate the [ChangeLog](https://github.com/nlohmann/json/blob/develop/ChangeLog.md)
 - [**Google Benchmark**](https://github.com/google/benchmark) to implement the benchmarks
 - [**Hedley**](https://nemequ.github.io/hedley/) to avoid re-inventing several compiler-agnostic feature macros
 - [**include-what-you-use**](https://include-what-you-use.org) to check the included headers