Automatic merge from submit-queue (batch tested with PRs 49615, 49321, 49982, 49788, 50355)
Simplify hack/verify-flags-underscore.py
**What this PR does / why we need it**:
This PR removes the need for `hack/verify-flags/known-flags.txt` and verify-flags-underscore.py will always parse the flags from go files to check if they have underscore.
It is much faster compared to earlier checks and it does its job to check for underscore in flags.
Now:
```
# time ./hack/verify-flags-underscore.py
real 0m1.638s
user 0m1.560s
sys 0m0.076s
```
Before:
```
# time ./hack/verify-flags-underscore.py
real 0m22.585s
user 0m22.464s
sys 0m0.112s
```
It has become a pain to keep adding new flag to `known-flags.txt` whenever a new flag is introduced. with this PR this is step is not required anymore.
**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes#40329#50319
**Special notes for your reviewer**:
**Release note**:
```
NONE
```
/cc @fejta @mtaufen
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