chore(docs): update the link to BCP 47

[xfq]

Describe the PR

tools.ietf.org is no longer to be relied on per https://mailarchive.ietf.org/arch/msg/tools-discuss/oYrAxb3KayPzZ4SNB1DVZTDPPNo/

AFAIK, rfc-editor.org is the recommended URL for RFC and BCP nowadays. Also, I changed the link from RFC 5646 to BCP 47 (see https://www.w3.org/TR/international-specs/#lang_bcp_not_rfc ).

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• Feature (adds a new feature to BootstrapVue) - feat(...), requires a minor version update
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• ARIA accessibility (fixes or improves ARIA accessibility) - fix(...), requires a patch or minor version update
• Documentation update (improves documentation or typo fixes) - chore(docs), requires a patch version update
• Other (please describe)

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• No
• Yes (please describe since breaking changes require a minor version update)

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Merging #7009 (c60304b) into dev (3e29c63) will not change coverage.
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[VividLemon]

If the recommended standard has changed, then that would assume that there is something on the new that is not supported by bootstrap-vue. You may want to explain a bit more about the scope of what this change is. To clarify, I am fine if this is only a docs change on their part. However, if the spec changed, and we are supposed to be supporting new tags, then linking to some different docs wouldn't be appropriate as it would be misleading.

[xfq]

This PR just uses the recommended URL of the IETF language tag standard, so if the spec is updated, we don't need to update the URL. This is also the URL recommended by W3C (for many years):

The link to and name of BCP 47 was created specifically so that there is an unchanging reference to the definition of Tags for the Identification of Languages. RFCs 1766, 3066, 4646 were previous (superseded) versions and 5646 is the current version of BCP 47.

In terms of content, RFC 5646 is just the core of BCP 47, so bootstrap-vue doesn't need to update any code. It doesn't introduce new language tags either, because the language subtags are managed by the IANA Subtag Registry.