Backport #12251: Update cabal-install 3.18.1.0 release notes

[mergify]

As discussed in today's call.

Warn users about a number of issues early 3.18 adopters found, and how to work around them.

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• Patches conform to the coding conventions.
• [ ] Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions). n/a

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This is an automatic backport of pull request #12251 done by [Mergify](https://mergify.com).

[ulysses4ever]

i thought backports need only one approve... but there's always some funny condition that i forget

[Mikolaj]

CI failure to investigate later (for now, I restarted the jobs):

2026-08-21T15:32:58.4191520Z       syncSourceRepos:                                                          FAIL (43.40s)
2026-08-21T15:32:58.4193170Z         *** Failed! Exception: 'ExitFailure 128' (after 6 tests):
2026-08-21T15:32:58.4193580Z         RepoDirSet 3
2026-08-21T15:32:58.4193850Z         SyncTargetIterations 6
2026-08-21T15:32:58.4194270Z         PrngSeed 6241857598215302032
2026-08-21T15:32:58.4195690Z         BranchingRepoRecipe [Left (TaggedCommits "tag_E" [Commit [Left (FileUpdate "file/A" "\\Bd;UTy)w8"),Left (FileUpdate "file/B" "$H2%p7s^R%")],Commit [Left (FileUpdate "file/D" "}*HUKcC_-K")],Commit [Left (FileUpdate "file/E" "r}Q5y\\OtIE"),Left (FileUpdate "file/D" "`b.XuTx]^T"),Left (FileUpdate "file/C" "Su9Yngxt4i")]])]
2026-08-21T15:32:58.4197140Z         Use --quickcheck-replay="(SMGen 10466273035733678407 3519378012105765637,50)" to reproduce.
2026-08-21T15:32:58.4197740Z         Use -p '/git.syncSourceRepos/' to rerun this test only

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-08-21 18:18 UTC · Rule: default · triggered by rule Put pull requests in the rebase+merge queue
• ✅ Checks skipped · PR is already up-to-date
• ✅ Merged — 2026-08-21 18:31 UTC · at e302e3d1ea1404094b341ac0573235d58cfa3445 · merge

This pull request spent 12 minutes 30 seconds in the queue, including 9 seconds running CI.

Required conditions to merge

• #review-threads-unresolved = 0 [🛡 GitHub branch protection]
  • Backport #12251: Update cabal-install 3.18.1.0 release notes #12252
• github-review-approved [🛡 GitHub branch protection]
  • Backport #12251: Update cabal-install 3.18.1.0 release notes #12252
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