Add note about API changes in internal code

Author: sebastianbergmann

.github/ISSUE_TEMPLATE/1_BUG.md

@@ -9,6 +9,7 @@ labels: type/bug, version/9, version/10
 - Please do not report an issue if you are using a version of PHP that is not supported by the version of PHPUnit you are using. A list that shows which version of PHP is supported by which version of PHPUnit is available at https://phpunit.de/supported-versions.html.
 - Please do not report an issue if you are not using PHPUnit directly, but rather a third-party wrapper around it.
 - Please do not report an issue if you are using a third-party extension such as alternative output printers.
+- Please do not report an issue for API changes in internal code (see https://phpunit.de/backward-compatibility.html#internal-code).
 - Please fill in this template according to your issue.
 - Please keep the table shown below at the top of your issue.
 - Please include the output of "composer info | sort" if you installed PHPUnit using Composer.

.github/ISSUE_TEMPLATE/2_BUG_PHPUNIT_10.md

@@ -9,6 +9,7 @@ labels: type/bug, version/10
 - Please do not report an issue if you are using a version of PHP that is not supported by the version of PHPUnit you are using. A list that shows which version of PHP is supported by which version of PHPUnit is available at https://phpunit.de/supported-versions.html.
 - Please do not report an issue if you are not using PHPUnit directly, but rather a third-party wrapper around it.
 - Please do not report an issue if you are using a third-party extension such as alternative output printers.
+- Please do not report an issue for API changes in internal code (see https://phpunit.de/backward-compatibility.html#internal-code).
 - Please fill in this template according to your issue.
 - Please keep the table shown below at the top of your issue.
 - Please include the output of "composer info | sort" if you installed PHPUnit using Composer.

.github/ISSUE_TEMPLATE/3_BUG_PHPUNIT_9.md

@@ -9,6 +9,7 @@ labels: type/bug, version/9
 - Please do not report an issue if you are using a version of PHP that is not supported by the version of PHPUnit you are using. A list that shows which version of PHP is supported by which version of PHPUnit is available at https://phpunit.de/supported-versions.html.
 - Please do not report an issue if you are not using PHPUnit directly, but rather a third-party wrapper around it.
 - Please do not report an issue if you are using a third-party extension such as alternative output printers.
+- Please do not report an issue for API changes in internal code (see https://phpunit.de/backward-compatibility.html#internal-code).
 - Please fill in this template according to your issue.
 - Please keep the table shown below at the top of your issue.
 - Please include the output of "composer info | sort" if you installed PHPUnit using Composer.