[Remove] LegacyESVersion.V_7_6_* and V_7_7_* constants

[nknize]

Removes all usages of LegacyESVersion.V_7_6_ and LegacyESVersion.V_7_7_ version constants along with ancient API logic.

relates #2747

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/4952/
• CommitID: 166612b

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/4953/
• CommitID: bd98c64

[nknize]

Oh fun! Looks like we have a race condition in IndexShardIT.testIndexCanChangeCustomDataPath.

deleteReaderFiles must be getting called async before the test finishes copying.

[2022-10-19T08:21:35,640][ERROR][o.o.i.s.IndexShardIT     ] [testIndexCanChangeCustomDataPath] Failed to copy data path directory
java.nio.file.NoSuchFileException: /var/jenkins/workspace/gradle-check/search/server/build/testrun/internalClusterTest/temp/org.opensearch.index.shard.IndexShardIT_B9157C425F286EBE-001/rxeTWWtHeW/start-KZvCXclcHt/0/ipMqHwdkRym5TzwG0a3ubg/0/translog/translog-3794781907109570892.ckp

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/4991/
• CommitID: 7cd881f

[codecov-commenter]

Codecov Report

Merging #4837 (7cd881f) into main (ec34737) will increase coverage by 0.06%.
The diff coverage is 68.32%.

@@             Coverage Diff              @@
##               main    #4837      +/-   ##
============================================
+ Coverage     70.70%   70.77%   +0.06%     
+ Complexity    57852    57842      -10     
============================================
  Files          4689     4689              
  Lines        277080   276866     -214     
  Branches      40302    40219      -83     
============================================
+ Hits         195917   195957      +40     
+ Misses        64935    64765     -170     
+ Partials      16228    16144      -84     

Impacted Files	Coverage Δ
...ch/analysis/common/CommonAnalysisModulePlugin.java	92.28% <0.00%> (-1.32%)	⬇️
.../src/main/java/org/opensearch/LegacyESVersion.java	65.00% <ø> (-0.72%)	⬇️
.../node/reload/NodesReloadSecureSettingsRequest.java	18.51% <0.00%> (+0.66%)	⬆️
...ster/snapshots/restore/RestoreSnapshotRequest.java	66.08% <ø> (+3.26%)	⬆️
...tion/admin/cluster/stats/ClusterStatsResponse.java	18.64% <0.00%> (+0.61%)	⬆️
.../src/main/java/org/opensearch/common/Rounding.java	86.72% <ø> (+0.17%)	⬆️
.../main/java/org/opensearch/index/engine/Engine.java	73.82% <ø> (-1.30%)	⬇️
...rg/opensearch/indices/analysis/AnalysisModule.java	94.23% <0.00%> (+2.64%)	⬆️
...ch/repositories/blobstore/BlobStoreRepository.java	64.84% <ø> (+0.85%)	⬆️
.../opensearch/transport/ProxyConnectionStrategy.java	64.06% <0.00%> (-22.97%)	⬇️
... and 520 more

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[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/5115/
• CommitID: a317763

[dblock]

I believe this has been fixed in #4270, rebase?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/5195/
• CommitID: 6bfadbb

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/6047/
• CommitID: 1f533a0
  Please examine the workflow log, locate, and copy-paste the failure below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: FAILURE ❌
• URL: https://build.ci.opensearch.org/job/gradle-check/6049/
• CommitID: 88f5daa
  Please examine the workflow log, locate, and copy-paste the failure below, then iterate to green.
  Is the failure a flaky test unrelated to your change?

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/6050/
• CommitID: 2692841