Require sebastian/diff 9

[Orest-Divintari]

Raises the minimum for sebastian/diff to ^9.0.

Dropping 7 and 8 means the code no longer has to work across three major versions of the diff API. Concretely, StrictUnifiedDiffOutputBuilder can take the header verbatim through the header option, which only exists from 8.3 on; #93 had to reconstruct the same string through fromFile/toFile to stay compatible with the older range. I have left that call as it is here, so this PR is only the constraint change — happy to simplify it in a follow-up if you would like.

The dev floors have to move too

CI resolves lowest as well as highest, and on the lowest leg the old dev floors are unsatisfiable against a root requirement of ^9.0:

package	floor before	floor now	why
phpunit/phpunit	^13.1.7	^13.2	13.1.14 requires sebastian/diff ^8.3.0; 13.2.0 is the first to require ^9.0
friendsofphp/php-cs-fixer	^3.95.1	^3.95.18	3.95.1 allows ^4.0.6 || … || ^8.0; 3.95.18 is the first to add ^9.0

Verified: composer update --prefer-lowest --dry-run now resolves, taking PHPUnit down to exactly 13.2.0 with sebastian/diff staying at 9.

One knock-on change

PHPUnit 13.2 soft-deprecates expectExceptionMessage(), and phpstan-deprecation-rules reports it, so the two call sites in tests/ move to expectExceptionMessageIsOrContains(). Without that, phpstan analyse fails as soon as the floor allows 13.2. Kept as its own commit.

Verified

• vendor/bin/phpunit — 185 tests, 667 assertions, OK
• vendor/bin/php-cs-fixer check — clean, 0 of 211 files need fixing
• vendor/bin/phpstan analyse — no errors
• composer update --prefer-lowest --dry-run — resolves, no conflict

[ruudk]

Thanks!