DOC Add changelog item for IsFirst/IsLast in elemental

[kinglozzer]

Description

Docs update for silverstripe/silverstripe-elemental#1275

Issues

silverstripe/silverstripe-elemental#1274

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[GuySartorelli]

For future reference the checkbox is there for the committer (you in this case) to fill in, as a way to double check everything is ready to be reviewed.
I've ticked the boxes this time around.

[GuySartorelli]

Lets use the same language as the other deprecations in the list.

Suggested change

	- For `BaseElement::First()` and `BaseElement::Last()` in [`dnadesign/silverstripe-elemental`](https://github.com/silverstripe/silverstripe-elemental) have been deprecated and `IsFirst()` and `IsLast()` have been added to replace them. This is to more closely match core template loop behaviour where `$IsFirst` and `$IsLast` have been used since CMS 5. These methods are commonly used in element templates, e.g. `<% if $First %>`/`<% if $Last %>`: please update usages to `<% if $IsFirst %>`/`<% if $IsLast %>`.
	- [`BaseElement::First()`](api:DNADesign\Elemental\Models\BaseElement::First()) has been deprecated. Use [`BaseElement::IsFirst()`](api:DNADesign\Elemental\Models\BaseElement::IsFirst()) instead.
	- [`BaseElement::Last()`](api:DNADesign\Elemental\Models\BaseElement::Last()) has been deprecated. Use [`BaseElement::IsLast()`](api:DNADesign\Elemental\Models\BaseElement::IsLast()) instead.