Fix PHP 8.1 compatibility issues

The module has some compatibility problems with PHP 8.1 which were identified in the Date module.

Date.DateMigrateTestCase
✗	
include_once
exception: [Deprecated] Line 15 of sites/all/modules/migrate/includes/source.inc:
Return type of MigrateSource::current() should either be compatible with Iterator::current(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 15 of sites/all/modules/migrate/includes/source.inc:
Return type of MigrateSource::next() should either be compatible with Iterator::next(): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 15 of sites/all/modules/migrate/includes/source.inc:
Return type of MigrateSource::key() should either be compatible with Iterator::key(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 15 of sites/all/modules/migrate/includes/source.inc:
Return type of MigrateSource::valid() should either be compatible with Iterator::valid(): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 15 of sites/all/modules/migrate/includes/source.inc:
Return type of MigrateSource::rewind() should either be compatible with Iterator::rewind(): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 864 of sites/all/modules/migrate/plugins/sources/xml.inc:
Return type of MigrateXMLReader::current() should either be compatible with Iterator::current(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 864 of sites/all/modules/migrate/plugins/sources/xml.inc:
Return type of MigrateXMLReader::next() should either be compatible with Iterator::next(): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 864 of sites/all/modules/migrate/plugins/sources/xml.inc:
Return type of MigrateXMLReader::key() should either be compatible with Iterator::key(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 864 of sites/all/modules/migrate/plugins/sources/xml.inc:
Return type of MigrateXMLReader::valid() should either be compatible with Iterator::valid(): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 864 of sites/all/modules/migrate/plugins/sources/xml.inc:
Return type of MigrateXMLReader::rewind() should either be compatible with Iterator::rewind(): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 3567 of includes/bootstrap.inc:
Return type of MigrateSQLMap::current() should either be compatible with Iterator::current(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 3567 of includes/bootstrap.inc:
Return type of MigrateSQLMap::next() should either be compatible with Iterator::next(): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 3567 of includes/bootstrap.inc:
Return type of MigrateSQLMap::key() should either be compatible with Iterator::key(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 3567 of includes/bootstrap.inc:
Return type of MigrateSQLMap::valid() should either be compatible with Iterator::valid(): bool, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

exception: [Deprecated] Line 3567 of includes/bootstrap.inc:
Return type of MigrateSQLMap::rewind() should either be compatible with Iterator::rewind(): void, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice