README: update for libicu dependency

[jcsp]

Problem

In 83e7e5d dependencies were only updated for Mac users. Without libicu, postgres 16 build fails.

Summary of changes

Update dependencies on Ubuntu and fedora to include libicu.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

2496 tests run: 2380 passed, 0 failed, 116 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_partial_evict_tenant: debug
• test_get_tenant_size_with_multiple_branches: debug
• test_wal_lagging: release

Postgres 14

• test_get_tenant_size_with_multiple_branches: debug

Code coverage (full report)

• functions: 53.0% (7796 of 14702 functions)
• lines: 81.1% (45443 of 56019 lines)

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_{The comment gets automatically updated with the latest test results
e067329 at 2023-09-20T16:05:44.218Z :recycle:}

[arpad-m]

I did not have to install libicu, but it is installed on my system for some reason. Could not find the package in history.log to find when I installed it.

[arpad-m]

Merged by myself because of hardware issue for @jcsp.