pd: update the default value of read-base-cost

[lilin90]

What is changed, added or deleted? (Required)

Updates the default value of read-base-cost from 0.25 to 0.125 for v7.5 and later versions

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

Tips for choosing the affected version(s):

By default, CHOOSE MASTER ONLY so your changes will be applied to the next TiDB major or minor releases. If your PR involves a product feature behavior change or a compatibility change, CHOOSE THE AFFECTED RELEASE BRANCH(ES) AND MASTER.

For details, see tips for choosing the affected versions (in Chinese).

• master (the latest development version)
• v9.0 (TiDB 9.0 versions)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)

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

• This PR is translated from:
• Other reference link(s): https://github.com/tikv/pd/blob/17f3ae3609fc50bae07e8783c0a373419b7daf9b/client/resource_group/controller/config.go#L66

Do your changes match any of the following descriptions?

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

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[lilin90]

/approve

[ti-chi-bot]

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[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.5: #19722.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.4: #19723.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #19724.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.1: #19725.

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.3: #19726.