sorter(ticdc): use correct cgroup memory limit in some scenarios (#8589)

[ti-chi-bot]

This is an automated cherry-pick of #8589

What problem does this PR solve?

Issue Number: close #8588

What is changed and how it works?

Use a better way to fetch cgroup memory limit.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

Have been verified in the following scenarios

• systemd cgroup enabled
• systemd cgroup not enabled
• docker run container under containerd-shim-runc-v2 with memory limit
• docker run container under containerd-shim-runc-v2 without memory limit
• K8s starts pod with memory limit
• K8s starts pod without memory limit

Questions

Will it cause performance regression or break compatibility?

Do you need to update user documentation, design documentation or monitoring documentation?

Release note

Use correct cgroup memory limit in sorter engine under some scenarios.

[ti-chi-bot]

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[ti-chi-bot]

@ti-chi-bot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-7.0@a544daa). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head bbf462c differs from pull request most recent head b08916c. Consider uploading reports for the commit b08916c to get more accurate results

Additional details and impacted files

Flag	Coverage Δ
cdc	66.3506% <0.0000%> (?)
dm	53.4119% <0.0000%> (?)
engine	65.4295% <0.0000%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

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