Prepare release 2.6.0 (part of #775)

[hartwork]

Part of #775, no ETA

Known missing pull requests as of the moment, need to be added (if merged):

• Grow buffer based on current size #771
• tests: Rename fail_unless to assert_true for clarity #774
• Prepare release 2.6.0 (part of #775) #776
• [CVE-2023-52426] Fix issues for compilation with XML_DTD undefined #777
• doc/reference.html: Add HTML anchors to definition of XML_* macros #779
• xmlparse.c: Fix NULL pointer dereference in setContext via XML_ExternalEntityParserCreate for XML_DTD undefined #780
• tests/acc_tests.c: Move second BOM accounting test to the right place #781
• tests/handlers.c: Drop mistaken unused variable marker #782
• tests: Stop switching between XML_Parse and _XML_Parse_SINGLE_BYTES #783
• tests/handlers.h: Drop unused type XmlParseFunction #784
• win32|cmake: Fix FileVersion and ProductVersion in DLL version info #785
• tests: Use NAN and INFINITY of C99 and C++11 #786
• tests: Migrate more tests to variable chunk size parsing #787
• tests: Narrow test_buffer_can_grow_to_max mingw allocation workaround at compile time #788
• [CVE-2023-52425] Speed up parsing of big tokens #789
• CMake: Build tests/benchmark/benchmark.c for EXPAT_BUILD_TESTS #790
• autotools: Sync CMake templates with CMake 3.26 #792
• CMake|Autotools: Fix PACKAGE_BUGREPORT variable to something working #793
• Autotools: Make installation of shipped doc/xmlwf.1 independent of docbook2man availability #795
• Make CI cover Clang's Control Flow Integrity sanitizer #796
• fuzz: improve coverage #797
• Make GitHub Actions enforce clang-tidy clean code + address current clang-tidy warnings #798
• Make CI run fuzzing regression tests (fixes #367) #799
• clang-tidy: Address warnings readability-avoid-const-params-in-decls and readability-named-parameter #800
• fuzz: Improve coverage by maybe stopping the parser #801
• Fix Cppcheck CI for Cppcheck 2.13.0 #803
• Define XML_STATIC in pkg-config file when building a static expat #805
• CI: Upgrade to clang-tidy 18 #808
• CI: Upgrade to clang-format 18 #809
• CI: Upgrade to Clang 18 (except clang-tidy and clang-format) #810
• Protect against closing entities out of order (fixes #812) #813
• tests: Fix CLOCKS_PER_SEC guard for arm64 FreeBSD reality #814
• pkg-config: Add missing -DXML_STATIC for Windows (alternative to #805) #815

Known missing issues as of the moment, need to be added:

• Suggest fuzzer for XML_Parse() and XML_GetCurrentLineNumber(). #660
• Expat can be made to close an open entity that is not at the top of the stack of open entities #812