Drop old language versions support in Functions v4

In Functions v4, drop support for the following language versions:
- **Node.js**
- Version **10**
- Community EOL: April 30, 2021
- Version **12**
- Community EOL: April 30, 2022
- **PowerShell**
- Version **6**
- Community EOL: September 4, 2020
- **Python**
- Version **3.6**
- Community EOL: December 23, 2021

All these versions are currently supported by Functions v3, and they must continue being supported until the date promised by Azure Functions, even if this date exceeds the community EOL date. This promise, however, does *not* imply that these language versions must be supported by Functions v4 *as well*. As long as Functions v3 is not deprecated, it serves the purpose of fulfilling the promise. So, it is important to make sure that Functions v3 is not deprecated earlier than the end of support dates for each mentioned language version.

Here is how the result will look like:

| Language | v1 | v2 | v3 | v4 |
|-----------|-----|----------|---------------|----------------------|
| Java | N/A | 8 | 11, 8 | 11, 8 |
| Node.js | 6 | 10, 8 | 14, 12, 10 | 16¹, 14 |
| PowerShell| N/A | 6 | 7.0, 6 | 7.2¹, 7.0 |
| Python | N/A | 3.7, 3.6 | 3.9¹, 3.8, 3.7, 3.6 | 3.10¹, 3.9¹, 3.8, 3.7 |

¹ Support of these versions in Azure Functions is expected by the end of 2021

When migrating Functions apps from v3 to v4, users will have to explicitly specify a new language version and review, test, and potentially modify their code.

See also:
- [Azure Functions language runtime support policy | Microsoft Docs](https://docs.microsoft.com/en-us/azure/azure-functions/language-support-policy)
- [Supported languages in Azure Functions | Microsoft Docs](https://docs.microsoft.com/en-us/azure/azure-functions/supported-languages)
- Internal: [Support and Deprecation Policies for Azure Functions.docx](https://microsoft-my.sharepoint.com/:w:/p/shinardi/EbMEwwjSULxBsSXtbaIoXqcBhEXZ2qJrKphgnwodxOsC7Q?e=4mfTvi)

### Motivation
- Reduce maintenance burden for Azure Functions engineers.
- Reduce the footprint of language workers to make them more effiecient.
- Encourage users to migrate to supported language versions.

### Impact
Functions v3 stats:
- **Node.js**
- Version **10**: ~22% of apps
- Version **12**: ~37% of apps
- **PowerShell**
- Version **6**: ~34% of apps
- **Python**
- Version **3.6**: ~17% of apps

### Compat-mode support
Introducing a feature flag to enable old language versions in Functions v4 would defeat the purpose of the change.

### Alternatives

#### Alternative 1: Do not drop support of old language versions
Keeping support for old language versions in Functions v4 would mean not getting the benefits described in *Motivation*.

#### Alternative 2: Auto-migrate apps to newer language versions
Language owners frequently introduce breaking changes in newer language versions. As a result, some Functions app code is expected to either stop working or produce unexpected and undesired behavior. In order to attract users attention to this fact and invite them to review and test their code, we should make sure the language version upgrade is performed by users explicitly.

#### Alternative 3: Drop support for older language versions even more aggressively
- Java
- We cannot drop Java 8 as the vast majority of Functions Java apps use this version, and they are expected to use it for the foreseeable future.
- Node.js
- We cannot drop Node.js 14 because its community EOL is April 30, 2023, which is later than the projected Functions v3 deprecation date.
- PowerShell
- We cannot drop PowerShell 7.0 because PowerShell 7.2 support cannot be added by the expected Functions v4 GA release date, so dropping 7.0 would mean that Functions v4 would GA without *any* version of PowerShell.
- Python
- We cannot drop Python 3.7 because its community EOL is June 27, 2023, which is later than the projected Functions v3 deprecation date.

### Detection
- Functions Host and language workers already emit error messages on invalid language versions.
- We should also make sure we have detectors notifying of this error and providing migration guidance.

### Support
Support needs to be aware of the language version support matrix.

### Documentation
The following documentation will need to be updated:
- [Supported languages in Azure Functions | Microsoft Docs](https://docs.microsoft.com/en-us/azure/azure-functions/supported-languages)
- [Node version](https://docs.microsoft.com/en-us/azure/azure-functions/functions-reference-node?tabs=v2#node-version)
- [PowerShell versions](https://docs.microsoft.com/en-us/azure/azure-functions/functions-reference-powershell?tabs=portal#powershell-versions)
- [Python version](https://docs.microsoft.com/en-us/azure/azure-functions/functions-reference-python?tabs=azurecli-linux%2Capplication-level#python-version)

### Components impacted
- Language workers
- Stacks API definition
- Tooling
- Az CLI
- Az PowerShell

### Performance
No performance impact.