resource group: change '' to the default resource group

[hfxsd]

What is changed, added or deleted? (Required)

As tittle

Which TiDB version(s) do your changes apply to? (Required)

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What is the related PR or file link(s)?

• This PR is translated from:resource_control: update bind to default resource group docs-cn#14315
• Other reference link(s):

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• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[glorv]

LGTM

[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-06-28 06:12:59.399213561 +0000 UTC m=+774544.800464010: ☑️ agreed by ran-huang.

[hfxsd]

/approve

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.1: #14010.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.2: #14011.