Upated: Version bump for development of next release to 6.0.13

Merge pull request #61 from se7enxweb/upgrade-phpunit-tests-to-phpunit10

Upgrade Exponential Support for phpunit tests from broken 3.7.x support to working phpunit v10 support

Merge pull request #60 from se7enxweb/security-hardening-exp-6013

Security Hardening Exponential 6.0.13 Kernel and Lib Libraries

DOC: Document PHP version compatibility — patch set does not raise minimum PHP version

Adds §14 'PHP Version Compatibility' to hardening.md and a new row to the
Scope table, clearly recording that the 6.0.13 security patch set does not
raise the project's declared PHP minimum (PHP 8.1).

New content covers:
- Per-file breakdown of the most modern PHP construct used in each patched file
- Most modern construct across the entire patch set: PHP 7.1 (nullable type
  hints in ezorder.php; short array destructuring in eztimetype.php)
- Clarification that the PHP84-tagged commits suppress deprecations on 8.4+
  but use only PHP 7.1 syntax and run identically on PHP 8.1–8.3
- A five-row summary table answering the questions a reviewers will ask:
  Does this raise the minimum? No. Current minimum? 8.1. Oldest compatible? 7.1.
  PHP 8.4 syntax used? No. Safe for 8.1/8.2/8.3? Yes.

DOC [SEC-01..06, NUL-01..14, PHP-01..03, UND-01..03, LOG-01..02, SET-01..06, KNT-01..12, IMP-01..02, PRG-01]: Add security hardening reference — hardening.md (1,176 lines)

Adds doc/bc/6.0/hardening.md — a comprehensive reference covering every defect
fixed in Exponential 6.0.13-alpha2 across 38+ kernel and lib source files.

Structure:
  §1  Overview — 6 attack surfaces, 32 defensive guards, 2 compat fixes across 38 files
  §2  SQL / shell injection (SEC-01..06) — ezrole, ezcontentobjecttreenode, ezsendmail, ezgzip
  §3  Undefined offset guards (UND-01..03)
  §4  Logic corrections (LOG-01..02)
  §5  Null dereference guards (NUL-01..14)
  §6  PHP 8.4 compatibility (PHP-01..03)
  §7  Incomplete SOAP stubs (IMP-01..02)
  §8  Setup wizard hardening (SET-01..06)
  §9  Kernel/content null safety (KNT-01..12)
  §10 preg_replace /e removal (PRG-01)
  §11 Test matrix — which security tests cover which issues
  §12 Pre-deploy checklist

Each section contains: root cause, attack vector or failure mode, exact
before/after code fragment, and reference to the covering test (where applicable).

DOC rename: phpunitvXXXX.md → phpunitv10.md

Pin the PHPUnit migration guide to its actual version target.
The placeholder name 'phpunitvXXXX' was used during authoring;
the document covers the PHPUnit 3.7 → 10 migration specifically.
Rename to phpunitv10.md to make the version unambiguous in directory
listings and cross-references.

DOC: Add PHPUnit 3.7→10 migration guide (phpunitvXXXX.md)

Adds doc/bc/6.0/phpunitvXXXX.md — a 916-line step-by-step guide explaining
why every test silently failed on eZ Publish Legacy / Exponential 6.0.x and
exactly how to fix it for PHPUnit 10.

Sections:
  §1  Background — PHPUnit class name history (3.x PEAR → 6.x namespace → 10)
  §2  Broken file inventory — table of every broken PHPUnit_* reference
  §3  Fix 1 — tests/toolkit/ezptestcase.php (extends PHPUnit\Framework\TestCase)
  §4  Fix 2 — tests/toolkit/ezptestsuite.php
  §5  Fix 3 — tests/toolkit/ezptestregressionsuite.php (Warning class removed)
  §6  Fix 4 — tests/toolkit/ezptestrunner.php (PHPUnit_TextUI_Command→Application shim)
  §7  Fix 5 — tests/runtests.php (PHPUnit_Runner_Version→PHPUnit\Runner\Version)
  §8  Fix 6 — create phpunit.xml at project root
  §9  Fix 7 — create tests/bootstrap.php
  §10 Verification — copy-pasteable commands end to end
  §11 How security hardening tests plug in
  §12 Full before/after diff summary
  §13 FAQ

Companion to the TEST infrastructure commit on this branch which contains the
actual fixed toolkit files, phpunit.xml, tests/bootstrap.php, and security suite.