release-24.1: sql/parser: fix memory leak of scanned comments

[blathers-crl]

Backport 1/1 commits from #127711 on behalf of @mgartner.

/cc @cockroachdb/release

---

This commit fixes a memory leak that cause a session's
scanner.Scanner, which is embedded in a pgwire.conn via a
parser.Parser, to retain previously scanned SQL comments in all parsed
SQL statements. The scanner's slice of comments would continue to grow
as it parsed new SQL strings with comments, as long as the session
remained open. Also, the slice of comments holds references to the bytes
in pgwire read buffers, preventing those buffers from being GC'd.

Fixes #127710

Release note (bug fix): A bug has been fixed that caused a memory leak
when executing SQL statements with comments, for example,
SELECT /* comment */ 1;. Memory owned by a SQL session would continue
to grow as these types of statements were executed. The memory would
only be released when closing the SQL session. This bug has been present
since version 23.1.

---

Release justification: Low-risk fix for a memory leak.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
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Also, please add a brief release justification to the body of your PR to justify this
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[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[michae2]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @fqazi and @yuzefovich)

[yuzefovich]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @fqazi)