Merge pull request #15745 from mmozuras/remove_unused_separators_param

Remove unused param 'separators' from RouteSet#build_path
Change Http::Cache::SPECIAL_KEYS from Array to Set

Slightly improves performance, for example, a simple benchmark:

```ruby
require 'benchmark/ips'
require 'set'

SPECIAL_KEYS = %w[extras no-cache max-age public must-revalidate]
SPECIAL_KEYS_SET = Set.new(SPECIAL_KEYS)
directive = 'must-revalidate'

Benchmark.ips do |x|
  x.report('array') { SPECIAL_KEYS.include?(directive) }
  x.report('set') { SPECIAL_KEYS_SET.include?(directive) }
end
```

Output:

```
-------------------------------------
   array     67926 i/100ms
     set     74054 i/100ms
-------------------------------------
   array  2318423.4 (±2.8%) i/s -   11615346 in   5.014899s
     set  3387981.8 (±4.7%) i/s -   16958366 in   5.019355s
```
Correct typo, add test for validates_absence_of, correct method names
Merge pull request #15748 from akshay-vishnoi/activemodel

Correct typo, add test for validates_absence_of
Merge pull request #15723 from akshay-vishnoi/sql-correction

[ci skip] Use `an` for SQL
Merge pull request #15722 from akshay-vishnoi/spell-correct

[ci skip] /mysql/i -> MySQL, Spell correct in continuation to #15555
Merge pull request #15743 from tgxworld/remove_unused_parameters

Remove unused parameter.
Merge pull request #15744 from mmozuras/special_keys_set

Change Http::Cache::SPECIAL_KEYS from Array to Set
Always update counter caches in memory when adding records

Before, calling `size` would only work if it skipped the cache, and
would return a different result from the cache, but only if:

- The association was previously loaded
- Or you called size previously
- But only if the size was 0 when you called it

This ensures that the counter is appropriately updated in memory.
Merge pull request #13963 from lucas-clemente/pending_migrations

Skip migration check if adapter doesn't support it
Merge pull request #15684 from aditya-kapoor/add-tests-for-assert-valid-keys

Add test cases for Hash#asset_valid_keys
[ci skip] correct doc for serialization in accordance with API Doc Guide
Refactor in-place dirty checking to use the attribute object
Add important plugins as one of the tasks of the release

[ci skip]
Merge pull request #15755 from sgrif/sg-dirty-refactor

Refactor in-place dirty checking to use the attribute object
[ci skip] correct output for asset_helper methods
    see cc255d3
Merge pull request #15751 from aditya-kapoor/correct-serialization-doc

[ci skip] correct doc for serialization in accordance with API Doc Guide
:scissors: and :nail_care: from d60c405 [ci skip]
[ci skip] add API doc for AR Group.
Merge pull request #15756 from aditya-kapoor/add-api-doc-for-querying

[ci skip] add API doc for AR Group.