update ShufflingRef version of get_attesting_indices to Electra v1.5.… #18276

All 4 980 tests pass, 347 skipped in 57m 2s

      12 files ±0   1 822 suites ±0 57m 2s ⏱️ - 2m 11s
  5 327 tests ±0   4 980 ✔️ ±0 347 💤 ±0 0 ❌ ±0
29 521 runs ±0 29 077 ✔️ ±0 444 💤 ±0 0 ❌ ±0

Results for commit 0e86c0d. ± Comparison against earlier commit 1778979.