Replace deprecated String.prototype.substr()

[CommanderRoot]

String.prototype.substr() is deprecated so we replace it with String.prototype.slice() which works similarily but isn't deprecated.
.substr() probably isn't going away anytime soon but the change is trivial so it doesn't hurt to do it.

[kurkle]

Merged as this was not a breaking change. If we decide to release another minor for v1, the milestone of this pr needs to be updated.

[stockiNail]

@kurkle I have seen that this PR created a release note 1.4.1 even if tagged as 2.0.0: https://github.com/chartjs/chartjs-plugin-annotation/releases/tag/untagged-e554510da7259acef101

I don't know if this is correct and a normal behavior.

[kurkle]

release-drafter determines the version based on version resolver config:

chartjs-plugin-annotation/.github/release-drafter.yml

Lines 28 to 40 in 67989c8

	version-resolver:
	major:
	labels:
	- 'breaking change'
	minor:
	labels:
	- 'enhancement'
	patch:
	labels:
	- 'bug'
	- 'chore'
	- 'types'
	default: patch

so unless its a major or minor, its a patch :)

it will update after new commits are added.

[stockiNail]

@kurkle thank you very much for explanation and apologize for this stupid question. I'm still trying to understand how it works all GHA in place for this project.

[kurkle]

https://en.wikipedia.org/wiki/No_such_thing_as_a_stupid_question