Separate the Protocol 20 software release date from the Mainnet activation date

The February 5, 2024 entry was headed 'Soroban Phase 1 (Mainnet Edition)'.
Sibling headings use the Mainnet token for an activation date, so the
heading read as the Protocol 20 Mainnet activation date. Mainnet upgraded
on February 20, 2024 at 1700 UTC; February 5 is the release date of
Stellar Core v20.2.0. The heading also said Phase 1 while its own table
lists Phase 0 limits and fees.

Rename the heading to Phase 0, drop the Mainnet token, and record the
activation date in one line under the heading.

Author: kaankacar

docs/networks/software-versions.mdx

@@ -765,7 +765,9 @@ This is the first version of the Soroban SDK that supports protocol 21. It is ma
 - Revert "[Epic] Separating soroban-rpc to prepare for repo change"
 - [Epic] Separating soroban-rpc to prepare for repo change
 
-## Protocol 20: Soroban Phase 1 (Mainnet Edition) (February 5, 2024)
+## Protocol 20: Soroban Phase 0 (software release, February 5, 2024)
+
+February 5, 2024 is the release date of this software, not a network upgrade date. Mainnet activated Protocol 20 on February 20, 2024 at 1700 UTC and ran the Phase 0 limits and fees listed below. Read more in the [Protocol 20 Upgrade Guide](https://stellar.org/blog/developers/protocol-20-upgrade-guide).
 
 ### Software