docs(changelog): add entry for the PHP 8.3 migration (#41727)

docs(changelog): add entry for the PHP 8.3 migration (#41449)

Signed-off-by: Thomas Müller <1005065+DeepDiver1975@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: DeepDiver1975

changelog/unreleased/41449

@@ -0,0 +1,8 @@
+Change: Raise minimum PHP version to 8.3
+
+The minimum required PHP version is now 8.3. Numerous PHP 8.x deprecation
+warnings were resolved across the codebase, the mail subsystem was migrated
+to symfony/mailer, the unused ext-apc requirement was dropped, and the PHP
+version check now runs early in lib/base.php.
+
+https://github.com/owncloud/core/pull/41449