2.05 Testing: Perform 2.05 test fork on xenon / testnet

[pavitthrap]

Need to define BITCOIN_TESTNET_2_0_LAST_BLOCK_HEIGHT and update STACKS_EPOCHS_TESTNET in src/core/mod.rs.

[jcnelson]

Do this in tandem with #2878

[jcnelson]

Let's try and get this done by this Thursday at the latest (ideally Wednesday)

[kantai]

To recap the action items before a launch on testnet can occur:

• Review and Merge Stacks 2.05: make p2p network reject connections from 2.0 peers after the epoch switch #2925 -- requires two approvals (pinging @kantai and @gregorycoppola)
• Review and Merge Instantiate costs-2.clar with latest results from benchmarking #2924 -- requires two approvals (pinging @jcnelson and @gregorycoppola)
• Review and Merge Stacks 2.05: Add Epoch to Cost Estimate Key #2923 -- requires one more approval (pinging @jcnelson)
• Merge 2.05: make it so a block whose anchored parent is in a different epoch cannot confirm a microblock stream #2920 -- approved, should be ready to merge (pinging @jcnelson)
• Deploy next-costs branch to a testnet follower, confirm that it can follow the current testnet, reuse existing chainstate. This can be tested before the above PRs merge.
• Pick testnet block height, (2,104,380) and open configuration PR (@kantai)
• Review and Merge configuration PR
• Tag a next-costs build and deploy to testnet nodes

[kantai]

Testnet fork activated!

https://explorer.stacks.co/txid/0xef13fb69ad25f49400fad61d82fb678cb59f8e584acd70450acd84b3e49ef706?chain=testnet