Update Platformio.ini to PIO v4 requirements

[ascillato]

Description:

Changed env_default to default_envs as required by PIO 4 and as to avoid compilation warnings.

Related issue (if applicable): NA

Checklist:

• The pull request is done against the latest dev branch
• Only relevant files were touched
• Only one feature/fix was added per PR.
• The code change is tested and works on core 2.3.0, 2.4.2 and 2.5.2
• The code change pass travis tests. Your PR cannot be merged unless tests pass
• I accept the CLA.

[Jason2866]

With this change pio <4 will not compile Tasmota anymore
Without the PR pio 4 generates just a warning.

[ascillato]

Yes, you are right, but in next version of PIO, env_default will not longer be supported.

And also, as by default, PIO upgrades automatically, most users will have PIO 4.

[arendst]

Let's decide to use pio 4 and continue with this change.