Add 7.1.14 and 7.1.15 release notes

[jzhou77]

Documentation changes only.

Code-Reviewer Section

The general guidelines can be found here.

Please check each of the following things and check all boxes before accepting a PR.

• The PR has a description, explaining both the problem and the solution.
• The description mentions which forms of testing were done and the testing seems reasonable.
• Every function/class/actor that was touched is reasonably well documented.

For Release-Branches

If this PR is made against a release-branch, please also check the following:

• This change/bugfix is a cherry-pick from the next younger branch (younger release-branch or main if this is the youngest branch)
• There is a good reason why this PR needs to go into a release branch and this reason is documented (either in the description above or in a linked GitHub issue)

[fdb-windows-ci]

Doxense CI Report for Windows 10

• Commit ID: ef37bf3
• Result: ✔️ SUCCEEDED
• Build Logs (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr-macos on macOS BigSur 11.5.2

• Commit ID: ef37bf3
• Duration 0:40:59
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Logs (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr-cluster-tests on Linux CentOS 7

• Commit ID: ef37bf3
• Duration 0:44:56
• Result: ❌ FAILED
• Error: Error while executing command: tar -czf "${CODEBUILD_SRC_DIR}/packaging/docker/website/${FDB_VERSION}/fdb_${FDB_VERSION}.tar.gz" --directory "${CODEBUILD_SRC_DIR}/packaging/docker/website/${FDB_VERSION}" .. Reason: exit status 1
• Build Logs (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr on Linux CentOS 7

• Commit ID: ef37bf3
• Duration 1:10:39
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Logs (available for 30 days)