BREAKING CHANGE chore: upgrade to i18next v21

[Sayan751]

Even though the public API for aurelia-i18n does not change, i18next in v21.x introduces breaking changes that may bring surprises for aurelia users. Hence marking this as a BREAKING CHANGE. Major changes are:

• Plural suffix changed from _plural to _other.
• Missing interpolation variables are not replaced with empty string by default and you need to set the following initialization options: interpolation: { skipOnVariables: false }

More details can be found here: https://www.i18next.com/misc/migration-guide#v20.x.x-to-v21.0.0.

[coveralls]

Pull Request Test Coverage Report for Build 3519b320-27db-4d28-801c-60ea939e5efa

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 80.714%

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Totals
Change from base Build f5827372-46fe-46f2-bf28-3a5bc283c9c1:	0.0%
Covered Lines:	406
Relevant Lines:	477

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💛 - Coveralls