PHP 8 Usage of max() with potential string values

Problem/Motivation

In link.migrate.inc, prepare uses max() to determine $max_delta. This has unintended consequences in PHP 8, as string comparison has changed in the cases where max() compares integer and string values, which can result in a string value like 'target' being used as the $max_delta.

This results in memory exhaustion later in the function where it attempts for ($delta = 0; $delta <= $max_delta; $delta++):
Out of memory (allocated 21563441152) (tried to allocate 42949672960 bytes) in contrib/link/link.migrate.inc on line 124

This is related to #3354440: PHP 8 ValueError: max(): Argument #1 ($value) must contain at least one element in max() in link.migrate.inc but slightly different.

Steps to reproduce

Migrate link values with extra attributes, e.g. a target attribute or similar.

Proposed resolution

Check $max_delta value uses only integer keys.