Prepare 2.6.0-rc.1 (#13263)

Prepare 2.6.0-rc.1 release

* Set neard version to `2.6.0-rc.1`
* Set crates version to `0.30.0`
* Set voting date to Monday 2025-04-14 19:00 UTC - protocol upgrade will
happen 12-24 hours later, on 2025-04-15 between 7:00 and 19:00 UTC.

According to our estimation tool the upgrade will happen at 2025-04-15
10:48:15 UTC

```
$ python3 estimate_epoch_start_time.py --chain_id testnet --num_future_epochs 30 --voting_date "2025-04-14 19:00:00"
Epoch -1: 11 hours, 38 minutes
Epoch -2: 11 hours, 40 minutes
Epoch -3: 11 hours, 41 minutes
Epoch -4: 12 hours, 14 minutes

Exponential weighted average epoch length: 11 hours, 47 minutes
Predicted start of epoch 1: 2025-04-03 15:46:07 UTC+0000 Thursday
Predicted start of epoch 2: 2025-04-04 03:33:42 UTC+0000 Friday
Predicted start of epoch 3: 2025-04-04 15:21:17 UTC+0000 Friday
Predicted start of epoch 4: 2025-04-05 03:08:53 UTC+0000 Saturday
Predicted start of epoch 5: 2025-04-05 14:56:28 UTC+0000 Saturday
Predicted start of epoch 6: 2025-04-06 02:44:03 UTC+0000 Sunday
Predicted start of epoch 7: 2025-04-06 14:31:39 UTC+0000 Sunday
Predicted start of epoch 8: 2025-04-07 02:19:14 UTC+0000 Monday
Predicted start of epoch 9: 2025-04-07 14:06:49 UTC+0000 Monday
Predicted start of epoch 10: 2025-04-08 01:54:25 UTC+0000 Tuesday
Predicted start of epoch 11: 2025-04-08 13:42:00 UTC+0000 Tuesday
Predicted start of epoch 12: 2025-04-09 01:29:36 UTC+0000 Wednesday
Predicted start of epoch 13: 2025-04-09 13:17:11 UTC+0000 Wednesday
Predicted start of epoch 14: 2025-04-10 01:04:46 UTC+0000 Thursday
Predicted start of epoch 15: 2025-04-10 12:52:22 UTC+0000 Thursday
Predicted start of epoch 16: 2025-04-11 00:39:57 UTC+0000 Friday
Predicted start of epoch 17: 2025-04-11 12:27:32 UTC+0000 Friday
Predicted start of epoch 18: 2025-04-12 00:15:08 UTC+0000 Saturday
Predicted start of epoch 19: 2025-04-12 12:02:43 UTC+0000 Saturday
Predicted start of epoch 20: 2025-04-12 23:50:18 UTC+0000 Saturday
Predicted start of epoch 21: 2025-04-13 11:37:54 UTC+0000 Sunday
Predicted start of epoch 22: 2025-04-13 23:25:29 UTC+0000 Sunday
Predicted start of epoch 23: 2025-04-14 11:13:05 UTC+0000 Monday
Predicted start of epoch 24: 2025-04-14 23:00:40 UTC+0000 Monday
Predicted start of epoch 25: 2025-04-15 10:48:15 UTC+0000 Tuesday
Predicted start of epoch 26: 2025-04-15 22:35:51 UTC+0000 Tuesday
Predicted start of epoch 27: 2025-04-16 10:23:26 UTC+0000 Wednesday
Predicted start of epoch 28: 2025-04-16 22:11:01 UTC+0000 Wednesday
Predicted start of epoch 29: 2025-04-17 09:58:37 UTC+0000 Thursday
Predicted start of epoch 30: 2025-04-17 21:46:12 UTC+0000 Thursday

Voting date falls into epoch 23.
Protocol upgrade will happen at the start of epoch 25: 2025-04-15 10:48:15 UTC+0000 Tuesday
```

---------

Co-authored-by: Razvan Barbascu <[email protected]>

Author: jancionear