store/tikv: rename func OnSendFailForRegion to OnSendFailForTiFlash

[AndreMouche]

Signed-off-by: AndreMouche

What problem does this PR solve?

store/tikv/region_cache: rename function OnSendFailForRegion to OnSendFailForTiFlash since it is only for tiflash. So we can easily distinguish the function OnSendFailForTiFlash from OnSendFail.

Part of #22513

Check List

Tests

• Unit test

Release note

• No release note

[sticnarf]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• andylokandy
• sticnarf

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[sticnarf]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: afc8953

[AndreMouche]

/run-e2e-test

[AndreMouche]

/run-unit-test

[ti-chi-bot]

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