storage_scrubber: retry on index deletion failures

[jcsp]

Problem

In automated tests running on AWS S3, we frequently see scrubber failures when it can't delete an index.

location_conf_churn:
https://neon-github-public-dev.s3.amazonaws.com/reports/main/11076221056/index.html#/testresult/f89b1916b6a693e2

scrubber_physical_gc:
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9178/11074269153/index.html#/testresult/9885ed5aa0fe38b6

Summary of changes

Wrap index deletion in a backoff::retry

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]

5013 tests run: 4855 passed, 0 failed, 158 skipped (full report)

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Flaky tests (5)

Postgres 16

• test_slots_and_branching: release-x86-64, release-arm64
• test_subscriber_restart: release-x86-64

Postgres 14

• test_ondemand_wal_download_in_replication_slot_funcs: release-arm64
• test_readonly_node_gc: release-arm64

Code coverage* (full report)

• functions: 31.3% (7489 of 23890 functions)
• lines: 49.6% (60107 of 121209 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
ef3db50 at 2024-10-01T08:30:36.404Z :recycle:}