Set version to 3.0.0a0 and update release notes

[EwoutH]

This PR prepares the first Mesa 3.0 pre-release, by:

• Setting the version to 3.0.0a0
• Updating the release notes with 2.3.1 and 3.0.0a0 changelog

For maintainers, a draft release page can be found here.

Edit: 3.0.0a0 is released!

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
Schelling	small	🔵 -0.3% [-0.7%, +0.1%]	🔵 -0.3% [-0.6%, -0.1%]
Schelling	large	🔵 -0.4% [-1.1%, +0.3%]	🔵 -3.5% [-5.3%, -2.1%]
WolfSheep	small	🔵 -0.4% [-1.5%, +0.8%]	🔵 +0.3% [+0.0%, +0.6%]
WolfSheep	large	🔵 -1.0% [-1.7%, -0.4%]	🔵 -1.1% [-2.7%, +0.1%]
BoidFlockers	small	🔵 -1.1% [-1.9%, -0.3%]	🔵 +0.4% [-0.3%, +1.2%]
BoidFlockers	large	🔵 +0.0% [-0.4%, +0.5%]	🔵 +1.0% [+0.4%, +1.5%]