Epic: pageserver sharding tech debt

[jcsp]

Tasks

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1. pageserver: clean up ancestral layers after split, old index_part objects #7043
   2 of 3
   a/tech_debt c/storage/pageserver +2
   
2. neon_fixtures.py should use explicitly-set shard by default, even for default unsharded scenario.
3. TenantHarness should use a sharding-enabled state by default for unit tests
4. Compaction: figure out if gap detection needs to be made stripe-aware to avoid spuriously splitting layers
5. pageserver: update metrics for shard IDs #5953
   c/storage/pageserver t/feature +2
6. initdb uploads: these are written in a shard-naive path. Make sure we have a sharded deletion test that checks they're cleaned up.
7. page_service: more efficient page_service -> shard lookup #6037
   a/tech_debt c/storage/pageserver +2
8. pageserver: clean up a redundant tenant_id attribute #6280
   a/tech_debt a/test +2
9. pageserver: avoid calculating logical size on shards >0 #6307
   a/tech_debt c/storage/pageserver +2
10. Ensure that safekeeper is only getting logical size feedback from shard 0
11. Revise initdb handling on shard >0 #6649
    a/tech_debt c/storage/pageserver +2
    
12. pageserver: clean up sharding helper names #6072
    a/tech_debt c/storage/pageserver +2