C++20 compatibility: a lot of warnings

### Basic Infos

- [x] This issue complies with the [issue POLICY doc](https://github.com/esp8266/Arduino/blob/master/POLICY.md).
- [x] I have read the documentation at [readthedocs](https://arduino-esp8266.readthedocs.io/en/latest) and the issue is not addressed there.
- [ ] I have tested that the issue is present in current master branch (aka latest git).
- [x] I have searched the issue tracker for a similar issue.
- [x] If there is a stack dump, I have decoded it.
- [x] I have filled out all fields below.

#### Platform

- Hardware: [ESP-8266]
- Core Version: [v3.0.2]
- Development Env: [Platformio]
- Operating System: [MacOS]

### Settings in IDE

- Module: [nodemcuv2]

### Problem Description

I've set a flag to compile with the latest (available) standard: `-std=gnu++20`.

It works, but it produces a lot of warnings, for example:

```
.platformio/packages/framework-arduinoespressif8266/cores/esp8266/core_esp8266_si2c.cpp:576:33: warning: '++' expression of 'volatile'-qualified type is deprecated [-Wvolatile]
576 | twi_data = twi_txBuffer[twi_txBufferIndex++];
| ^~~~~~~~~~~~~~~~~
.platformio/packages/framework-arduinoespressif8266/cores/esp8266/core_esp8266_si2c.cpp:579:9: warning: '--' expression of 'volatile'-qualified type is deprecated [-Wvolatile]
579 | bitCount--;
| ^~~~~~~~
.platformio/packages/framework-arduinoespressif8266/cores/esp8266/core_esp8266_si2c.cpp:588:18: warning: compound assignment with 'volatile'-qualified left operand is deprecated [-Wvolatile]
588 | twi_data <<= 1;
| ~~~~~~~~~^~~~~
.platformio/packages/framework-arduinoespressif8266/cores/esp8266/core_esp8266_si2c.
```

They are all connected to the fact that starting with C++20, some of the `volatile` operations are deprecated: https://blog.feabhas.com/2021/05/modern-embedded-c-deprecation-of-volatile/

1. Is there a plan for better support of C++20?
2. Are you open for PRs in this matter?