Update outdated https://en.pingcap.com/ links

[dveeden]

What is changed, added or deleted? (Required)

Note: Another option would be to make en.pingcap.com a cname for www.pingcap.com in the DNS settings. I'm not sure which option is best.

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.

• master (the latest development version)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v8.0 (TiDB 8.0 versions)
• v7.6 (TiDB 7.6 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)
• v5.3 (TiDB 5.3 versions)
• v5.2 (TiDB 5.2 versions)
• v5.1 (TiDB 5.1 versions)

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

• This PR is translated from:
• Other reference link(s):

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

[lilin90]

/lgtm

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-8.0: #18124.

[ti-chi-bot]

In response to a cherrypick label: new pull request could not be created: failed to create pull request against pingcap/docs#release-7.5 from head ti-chi-bot:cherry-pick-18096-to-release-7.5: the GitHub API request returns a 403 error: {"message":"You have exceeded a secondary rate limit and have been temporarily blocked from content creation. Please retry your request again later. If you reach out to GitHub Support for help, please include the request ID 8E84:432C2:FB252D:1B6FAE0:66860DE3 and timestamp 2024-07-04 02:50:11 UTC.","documentation_url":"https://docs.github.com/rest/overview/rate-limits-for-the-rest-api#about-secondary-rate-limits","status":"403"}

[ti-chi-bot]

In response to a cherrypick label: new pull request created to branch release-7.5: #18125.