Account for 12 second block times in stable parachain confirmations

The stable parachain confirmations had been set to 600 parachain blocks. However, this was using 6-second block times that are standard in standalone mode. This proposal corrects the parachain confirmations to 300 blocks accounting for 12-second block times in the parachain mode.

The arguments are obtained as follows:

• storage key obtained by going to chain state -> btcRelay -> stableParachainConfirmations. It shows the encoded storage key.
• The value should be 300 (because 300 * 12 = 3600), which in hex is 0x12c. Encoded in 32 bit little endian this is 0x2c010000